Newly developed UV-curable inkjet technology
for Digital Inkjet Press “KM-1”
Toshiyuki Takabayashi , Hirotaka Iijima , Katsunori Goi , Mitsuru Obata ,Toshiyuki Mizutani , Hideo Watanabe , Tadashi Hirano

Abstract

Introduction

We have developed a new and unique UV-curable inkjet

Nowadays ICT, e.g. internet, SNS, and e-book, has

technology for a sheet-fed off-set like inkjet printer “KM-1,”

become popular and been expanded throughout thewhich will bring a new wave of digitalization into the com-world. Such technology does not need paper as amercial printing world. There were many technical challeng-

medium for communication and promotes so-called

es to realize both high-speed and high-quality recording“paperless” environment rapidly. Accordingly it forc-

process in a system. We studied the imaging process thor-

es the commercial printing market to transform; tooughly and reached a new and unique UV-curable ink formu-

adapt much effective communication, to change mar-lation and a process that can cure images at extremely highketing method from “mass-” to “one-to-one” commu-

speed with extremely high definition. To support the ink andnication, and ephemeralization. It is essential to real-the process, we also developed a variety of technologies

ize quick delivery, high-mix low-volume production,such as a new durable print head with high nozzle density,and personalization for such transformation.

an image correction system, and so on. Combined these new

In Drupa 2012 we proposed an answer for the trans-technologies, as a synergy of inks, print heads, and system,formation; a sheet-fed inkjet digital press “KM-1.” [1]we have succeeded to develop a new inkjet printing systemIts vision is “Real digitalization Pioneer for Commercial

for digital press market.

Printing.”
Fundamental requirements are:
1. Quality: Close to offset
2. Flexibility: Wide variety of paper is available
3. Duplex
4. Productivity
5. Utilizing existing peripheral equipment.
We aim to provide real Digitalization tool to theCommercial Printing Market. Its specifications are
summarized in Table 1.
Table 1 KM-1 specifications.

Printing typeTransportation
Speed
Number of colors
Number of heads
Resolutions
Maximum print media size
Media thickness
Media type
InkRIP＊ R&D Division, Inkjet Business Unit, Konica Minolta, Inc.; Hino, Tokyo, JAPAN46

KONICA MINOLTA TECHNOLOGY REPORT VOL. 13 (2016)

Specifications
Single pass inkjet printing
Gripper transportation
Simplex: 3,000 sheets/hour
Duplex: 1,500 sheets/hour
4 colors (Y, M, C, K)
2 Heads/module
8 Modules/color(Total 64 heads/color)
1,200 dpi x 1,200 dpi
585 x 750 mm
Simplex: 0.06 - 0.6 mm
Duplex: 0.06 - 0.45 mm
Coated paper
Non-coated paper
Embossed paper etc.
UV-curable ink
Full variable

One of the biggest challenges of the KM-1 was tothe “pinning.” Figure 3 illustrates the inhibition; there

develop a new UV-curable inkjet technology, which

is an induction period of ca. 0.5 seconds and thecan achieve extremely stable jetting performancepolymerization starts after the period. The landed

necessary for single-pass printing and aptitude forink droplets can easily start to coalesce together

wide range of paper without pre-conditioning liquid.

within such period and it is impossible to avoid theWe will report the technologies of the ink, printing

image deterioration in high-speed single-pass print-process, and print head of the KM-1.

ing technology.

Fig. 1 Structural Illustration of KM-1 System.

Achievement technology

Fig. 3 Induction Period of Radical Polymerization. FT-IR spectroscopy:
λ=1626cm-1

A pre-conditioner is another candidate. Some ionicmaterials can coagulate pigments of the water-based

Forming high definition image
In the commercial printing market, a wide variety

inks. Utilizing such materials as a pre-conditioner canof papers, absorbable or non-absorbable (coated pa-avoid dots coalescing and bleeding. Water-based inks

per, art paper, uncoated paper, embossed paper, etc.)

and pre-conditioning liquid, however, effects on someare used. When we print on a non-absorbable paper

papers to cause paper-deformation, curling and/orwith a conventional ink (non-adjusted/customized towaving, and it hinders simultaneous duplex printing.

non-absorbable papers), image deterioration is causedby dots coalescing and bleeding. Such deterioration< Waterbase >is emphasized with a single-pass printing technology
and the image becomes streaky.
< UV ink>

Fig. 4 UV ink minimizes effect on the paper.

Under these circumstances, we chose UV-curable
ink to avoid such paper-deformation. To avoid theFig. 2 Streaky Image by Dots Coalescing.

dots coalescing and the bleeding, we then developed

For avoiding the image deterioration, many ideas

was designed to have relatively higher viscosity athave been proposed. Utilizing UV-curable ink is one

room temperature and lower viscosity at elevatedcandidate. Irradiation of UV light just after landing oftemperature. The ink is heated inside the print head

the ink can “pin” the ink to avoid dots coalescing andto decrease the viscosity for jetting. When the ink

bleeding. With conventional radically polymerizable

droplets land on the paper, the temperature of the ink

UV-curing ink, however, its polymerization are easily

decreases and the viscosity increases rapidly. The

suffering from inhibition by oxygen that hinder

rapid viscosity change “pins” the dots and it avoids

a new ink formulation and curing process. The ink

KONICA MINOLTA TECHNOLOGY REPORT VOL. 13 (2016)47

the dots coalescing and the bleeding accordingly.

over a long period. It takes long time to confirm theWith the ink we can print at a high speed on a wide

validity of the selection.

variety of paper, absorbable or non-absorbable with

We tried “Pulse NMR Measurement” as an easyand short time method for evaluating the pigment

high definition image.

dispersion stability. Principle of measurement isbased on the different behavior under the change ofmagnetic field between the molecules adsorbed on
the particles and the free molecules in the bulk liquid. This effects can be detected by NMR relaxation
time measurements. We suppose that the molecular
movement of the dispersants steadily adsorbing onthe surface of the pigment particles are limited andNMR relaxation time becomes short. Conversely thefree dispersants in the dispersant medium have longrelaxation time.

Fig. 5 Making High Quality Image by Controlling Ink Viscosity.

The point of the development was to attain higher
stability at elevated temperature with keeping higher
curing speed. We designed new ink formulation tofulfil such characteristics. To jet the ink stably, wedeveloped a new print head that has heat resistancewith highly controlled jetting driver. Another challenge was to gain excellent glossiness. The rapid“pinning” can avoid the dots coalescing but can yieldFig. 6 Expectation of the NMR relaxation time of adsorbed and freedispersants.

bumpy surface that gives uneven gloss. We developed a new halftoning process to improve the gloss.

We experimented the combination of the pigment,Combined, we finally achieved to develop the innova-

the dispersants, and the dispersant medium accord-

tive and reliable digital inkjet printing system “KM-1.”

ing to Table 2 and compared the relaxation time ofEach Technology is described below in detail.

“Pulse NMR Measurement” with the fluctuation ofaverage particle diameter over as an actual dispersant stability.

Ink Technology
1. Development of Pigment Dispersion Stability
To attain higher stability at elevated temperature,

Table 2 E xperimented samples of the stability test
using NMR measurement.

we carried out thorough investigation of the dispersion-stability of pigment particles in ink.
It is said that the adsorbability of dispersant is an
important factor to control the stability. When thesurface of the pigment particles are adsorbed by thedispersant appropriately, the particles gain higherstability. It has been used the acid-base interaction
between the surface of the pigment particles and thedispersant to select the best dispersant for a givenpigment. The best dispersant has been seeking byThe results are shown Figure 7. The dispersant sta-measuring the acid- and base-values of the dispersantsbility had a strong correlation with the NMR relax-

and comparing them with those of the pigment. [2] It

ation time. More specifically if the samples had theis convenient way, however, it is essential to evaluate

short relaxation time (Tsp), the fluctuation of the av-

the dispersion behavior at elevated temperatureserage particle diameter of samples became small.

48

KONICA MINOLTA TECHNOLOGY REPORT VOL. 13 (2016)

excellent adhesion is, the higher speed is with lowlight. Combined the formulation that show excellent
performance of both shall have higher curing speed.
As a result, inks with oligomer-A, oligomer-C, andoligomer-E showed excellent performance.
Table 3 Result of curing speed tests.

Fig. 7 Correlation between NMR relaxation time and Dispersion stability.
Tsp=Ts/Tb, Ts: NMR relaxation time of sample, Tb: NMR relaxation
time of bulk liquid (dispersant and dispersant medium).

By using this evaluation method, we achieved toattain higher stability of the ink. It was stable evenafter storage for 12 months at 60 degrees C.

free
OligomerA
OligomerB
OligomerC
OligomerD
OligomerE
OligomerF
OligomerG

Fastness to rubbing by paper *1

Tape peel off test *2

Good
Excellent
Excellent
Excellent
Excellent
Excellent
Good
Poor

Poor
Good
Poor
Good
Poor
Good
Poor
Good

Hyperbranced-polymer
Hyperbranced-polymer
Polyester acrylate
Polyester acrylate
Epoxy acrylate
Urethane acrylate
Urethane acrylate

10 wt % contain : the total weight of ink is 100wt %*1: weight : 500g/cm 2, paper : OK Top Coat plus, Good is equal with oil off-set printings*2: tape : 3M scotch tape, paper : OK Top Coat plus, Good is not peel off from paper

We then evaluated “Dynamic Viscoelasticity” of thecured films with oligomer-A, C, and E, to access theflexibility. The thickness of the cured films were 20
μm. As shown in Figure 9, E"-values of the films weredifferent depending on the oligomers. We determined
the tanδ values and calculated the dynamic glasstransition temperature (Tg). Finally we chose the formulation with the oligomer-C that showed the lowest
Tg. It satisfied both the curing speed and the flexibilFig. 8 Stability of the inks at 60 degrees C.

ity. We achieved high productivity of 3,000 sheetsper-hour using UV-LED light with giving excellent
flexibility with this ink.

2. Ink formulation
The next target of our ink-development was to keep
higher curing speed. It is well known that formulating
multi-functional monomers are effective to improvethe speed due to higher ratio of cross-linkage in thecured ink. When the ratio of the multi-functionalmonomers increases, however, the flexibility of thecured ink decreases significantly. The cured film cancurl and can even be cracked by folding. [3] We thentried to formulate the ink with keeping both the speed
and the flexibility.
We prepared inks with 10wt % oligomers (=multifunctional monomers) according to Table 3, and eval-Fig. 9 Dynamic viscoelasticity measurement of cured film (thickness:20 μm).

Image correction systemuated scratch resistance and adhesion to the paper

The next challenge was to gain excellent gloss.

surface as a means of curing speed. We thought theWhen the ink was jetted and cured on a non-porous

scratch resistance corresponded to the curing speed

media such as a coated paper, the surface of the im-at the surface. The curing reaction at the surface isage became bumpy and gave uneven gloss by using ain competition with the oxygen-inhibition; the tough-conventional halftoning process. The gloss value waser the resistance is, the higher the speed is with oxy-strongly correlated with the dot pattern. The single-gen. We thought the adhesion to the paper surface

pass printing process emphasized the pattern because

corresponded to the curing speed at interface be-

it printed for just one direction. More specifically, thetween the ink-dots and the paper. Due to the absorp-

gloss value of the image in a parallel direction to thetion of pigment particles, the intensity of the UV-lighttransportation was relatively higher compared to theis quite low deep inside the ink-dots. The moregloss value in a perpendicular direction.

KONICA MINOLTA TECHNOLOGY REPORT VOL. 13 (2016)49

Gloss value of 60 degrees

50

Head technology

4030To achieve stable, accurate, and high-speed jetting

20at elevated temperature, we developed a new print

100transportation

head KM1800iSHC for “KM-1.”
020

40

60

Dot%

80100Fig. 10 Conventional Halftone.

To improve such gloss-unevenness, we developed
a new halftoning process that provided redundancy
in a dot arrangement and controlled the gloss value
by using multi-sized dot with keeping a dot coverageon paper. [4] By using this improved halftoning process, we achieved to reduce the differences of thegloss value between the perpendicular and parallel
direction in high-density dot-recording area. The ratioof the gloss value, perpendicular gloss/parallel gloss,
was improved from 2.2 to 1.2.

Fig. 13 Newly Developed Print head KM1800iSHC.
Table 4 KM1800iSHC Specifications.

Number of nozzles
Resolutions
ActuatorFrequency
Internal Heater
Fig. 11 Improved Halftone.

Specifications
1,800 nozzles
600 npi (Nozzle pitch 40.3μm)
Piezoelectric shear mode;
HA structure
41 kHz (2-Drop driving mode)Available

The print head KM1800iSHC is designed based on“HA” structure. [5] To control the influence of heat

During the development, we found another issue ofon the jetting performance, we carried out designing

the gloss. The gloss value increased rapidly in accor-of total heat conduction passage. We designed thedance with the dotrecording ratio from 30 % to 100 %driving circuit to control its heat generation, the in-by using the conventional halftoning process. Suchternal heater to maintain the ink temperature without

increase gave discontinuous gloss change and gaveaffecting to jetting, and the structure of the head touneven feelings to viewers. The new halftoning pro-exhaust heat to outside. We also designed the struc-cess made it possible to improve the issue. We canture to reduce the stress and the strain at the actuator

control the gloss value of the image in a certain range

from the heat.

by controlling the spatial frequency of the multi-sized
dot pattern.

Fig. 14 Simulation of stress and strain at the actuator/ink supply channel
under heating.

We developed a new line head module that can
mount 16 print heads in the length of 585 mm in
1,200 dpi resolution. The chassis is simple and rigid,
and has the head positioning structure which enabledprint heads to be mounted in a proper position simFig. 12 Control the gloss value of image by pattern-dithering.

50

ply and accurately.

KONICA MINOLTA TECHNOLOGY REPORT VOL. 13 (2016)

Fig. 15 Line Head module.

We also designed a new driving waveform and a
new structure of pressure chamber that enable highproductivity and stable jetting property.

Fig. 16 Simulation of the ink ejecting by newly developed driving waveform and pressure chamber.

Conclusion
Commercial printing market is now required drastic change to digital. There are many technical challenges to fulfill the requirement by high-speed singlepass inkjet. We want to open new world by the newly
developed UV-curable inkjet technology not only reported this report but also improved continuously.
Sheet-fed off-set like inkjet printer “KM-1” is “TheFirst Ball” and we continue to challenge ourselves.

References[1] M. Obata, T. Sugaya, T. Mizutani, H. Watanabe, T. Takabayashi,H. Iijima :Development of the 23”×29.5” Sheet-fed Inkjet
Press KM-1, Proceedings of Nip 30 and Digital Fabrication
2014, p. 372–374, 2014[2] T.Kuniyoshi and T.Kobayashi, Shikizai, 67[9], p547–554,
1994[3] Yasuo Yoshihiro, Takao Hiraoka, Mitsunobu Morita, SohNoguchi, A New UV Curable Inkjet Ink : Follow-up Report,
Proceedings of Nip 29, p. 395–397, 2013[4] JP 5691591[5] H. Watanabe, T. Okuno, S. Kawaguchi, H. Takamatsu, M.
Ueda :New Developments of Shear-Mode Piezo Inkjet
Heads for Industrial Printing Applications, Proceedings ofNip 27 and Digital Fabrication 2011, p. 282–285, 2011

Acknowledgment
Reprinted with permission of IS&T: The Society for Imaging
Science and Technology sole copyright owners of NIP31:
International Conference on Digital Printing Technologies andDigital Fabrication Technical Program, Abstracts, and USB
Proceedings.

KONICA MINOLTA TECHNOLOGY REPORT VOL. 13 (2016)51