U.S. patent application number 09/789053 was filed with the patent office on 2001-07-26 for process and device for curing uv printing ink.
Invention is credited to Schmitt, Peter.
Application Number | 20010009701 09/789053 |
Document ID | / |
Family ID | 7760479 |
Filed Date | 2001-07-26 |
United States Patent
Application |
20010009701 |
Kind Code |
A1 |
Schmitt, Peter |
July 26, 2001 |
Process and device for curing UV printing ink
Abstract
The invention concerns a process and a device for curing a UV
curing printing ink (14) on a printed material (9), wherein the
printing ink (14) is irradiated with UV light from a UV radiation
source (8). A low pressure gas discharge lamp (7) is proposed as UV
radiation source (8). The device in accordance with the invention
is characterized in that it comprises a stationary reflector (5)
having, in particular, a special reflecting layer with diffuse
reflecting material based on silicone rubber having diffuse
reflecting particle imbedded therein.
Inventors: |
Schmitt, Peter; (Wurzburg,
DE) |
Correspondence
Address: |
Ira J. Schaefer, Esq.
Chadbourne & Parke LLP
30 Rockefeller Plaza
New York
NY
10112
US
|
Family ID: |
7760479 |
Appl. No.: |
09/789053 |
Filed: |
February 20, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09789053 |
Feb 20, 2001 |
|
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08945614 |
Oct 27, 1997 |
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Current U.S.
Class: |
427/559 ;
427/558 |
Current CPC
Class: |
B41M 7/0081
20130101 |
Class at
Publication: |
427/559 ;
427/558 |
International
Class: |
B05D 003/06 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 27, 1995 |
DE |
195 15 462.2 |
Apr 25, 1996 |
DE |
PCT/DE96/00767 |
Claims
1. Process for curing a UV curing printing ink (14) on a printed
material (9), wherein the printing ink (14) is irradiated with UV
light from a UV radiation source (8), characterized in that, a low
pressure gas discharge lamp (7) is utilized as the UV radiation
source (8) which has a spectral radiation flux integrated over the
UV-B and UB-C region of more that 50% of the UV radiation flux,
preferentially more than 75% of the UV radiation flux.
2. The process of claim 1, characterized in that the maximum of the
spectral radiation distribution of the low pressure gas discharge
lamp (7) lies in the V-B or UV-C region.
3. The process of claim 1 or 2, characterized in that the
integrated spectral UV radiation flux, in particular the UV-C
radiation flux, of the low pressure gas discharge lamp (7) above a
wavelength of 190 nm, in particular above 240 nm, is in excess of
50%, preferentially more that 75%, of its UV radiation flux, in
particular of its UV-C radiation flux.
4. The process of one of the preceding claims, characterized in
that the integrated UV radiation intensity, in particular the
integrated UV-B and UV-C radiation intensity or, in particular, the
UV-C radiation intensity, is between 1 and 100 mW/cm.sup.2,
preferentially between 10 and 50 mW/cm.sup.2.
5. The process of one of the preceding claims, characterized in
that the printing ink (14) is not heated above 40.degree. C. during
the UV curing.
6. The process of one of the preceding claims, characterized in
that the printing ink (14) has high reactivity at room
temperature.
7. The process of one of the preceding claims, characterized in
that the time duration of irradiation for curing the printing ink
(14) is less than two seconds, preferentially less than one
second.
8. The process of one of the claims 1 through 7, characterized in
that the printing ink (14) comprises the following components: a)
one or more cycloaliphatic epoxy resins as curable fixing agent,
and b) one or more arylsulfonium salts as photo-initiator.
9. The process of claim 8, characterized in that component b)
contains a mixture of differing arylsulfonium salts.
10. Device for curing a UV curing printing ink (14) on a printed
material (9), by means of which the printing ink (14) is irradiated
with UV light from a UV radiation source (8), in particular for
carrying out the process according to one of the claims 1 through
9, characterized in that the UV radiation source (8) comprises a
low pressure gas discharge lamp (7) having a spectral radiation
flux integrated over the UV-B and UB-C region of more that 50% of
the UV radiation flux, preferentially more than 75% of the UV
radiation flux.
11. The device of claim 10, characterized in that the maximum of
the spectral radiation distribution of the low pressure gas
discharge lamp (7) lies in the UV-B or UV-C region.
12. The device of claim 10 or 11, characterized in that the
integrated UV radiation flux, in particular the Uv-C radiation
flux, of the low pressure gas discharge lamp (7) above a wavelength
of 190 nm, in particular above 240 nm, is in excess of 50%,
preferentially more that 75%, of its UV radiation flux, in
particular of its UV-C radiation flux.
13. The device of one of the preceding claims, characterized in
that it comprises a plurality, in particular more than four and
preferentially more than eight, of low pressure gas discharge lamps
(7).
14. The device of claim 13, characterized in that it comprises low
pressure gas discharge lamps (7) having mutually differing emission
spectra.
15. The device of one of the preceding claims, characterized in
that the electrical power consumption of the low pressure gas
discharge lamps (7) is between 0.2 and 2.5, preferentially between
0.5 and 1.0 watts per centimeter of wavelength.
16. The device of one of the preceding claims, characterized in
that the homogeneity of the UV radiation intensity on the printed
material (9), in particular the UV-B and/or the UV-C intensity, in
the region effective for curing the printing ink (14) is
sufficiently high that the irradiation intensity deviates from an
average value by less than 30%, preferentially by less that
20%.
17. The device of one of the preceding claims, characterized in
that it comprises a plurality of mutually adjacent low pressure gas
discharge lamps (7), wherein the separation between the low
pressure gas discharge lamps (7) is not more then 30%,
preferentially is not more than 20%, of the diameter of the low
pressure gas discharge lamp (7) bulbs (16).
18. The device of one of the preceding claims, characterized in
that it comprises a plurality of U-shaped low pressure gas
discharge lamps (7) disposed in mutual adjacency at the parallel
lengthwise sides of the U-shape, wherein the low pressure gas
discharge lamps (7) are disposed in alternating opposite
directions.
19. The device of one of the preceding claims, characterized in
that the separation between the low pressure gas discharge lamps
(7) and the printed material (9) is less than 5 cm and/or more than
1 cm.
20. The device of one of the preceding claims, characterized in
that the low pressure gas discharge lamp (7) is a mercury vapor
lamp or an amalgam lamp.
21. The device of one of the preceding claims, characterized in
that it comprises a reflector (5) to reflect the UV light emitted
from the low pressure gas discharge lamps (7) onto the curing
printing ink (14).
22. The device of claim 21, characterized in that the reflector (5)
is stationary.
23. The device of claim 21 or 22, characterized in that the
reflector (5) comprises a dielectric mirrored layer and/or a
reflecting layer made from an optically diffuse reflecting material
(1).
24. The device of claim 23, characterized in that the optically
diffuse reflecting material (1) comprises a matrix of transparent
matrix material consisting essentially of a curable silicone rubber
in which diffuse reflecting particles are imbedded.
Description
[0001] The invention concerns a process for curing a UV curable
printing ink on a printed material, wherein the printing ink is
irradiated with UV light from a UV radiation source. The invention
also concerns an associated device for irradiating the printing ink
with UV light.
[0002] UV curing printing inks contain low amounts of solvent or no
solvent, cure when irradiated, and have recently become
increasingly important. This is due to the high energy of UV
radiation which is particularly advantageous in high speed printing
of printed materials, in particular for flat bed printing and
letter press printing. They also have practical advantages from a
technical applications point of view compared to solvent-containing
ink e.g. with regard to their working lifetime, solvent related
environmental pollution, and waste disposal.
[0003] UV curing printing inks have a UV curable fixing agent
system comprising a polymerizing fixing agent or mixture of fixing
agents and one or more associated photo-initiators. The
polymerization or cross-linking can be triggered by UV irradiation
to cure the ink. One differentiates between radical-induced and
cationic polymerization. Conventional radical-induced polymerizing
fixing agents are based on acrylates, whereas the cationic
polymerizing ones are characterized by acid release during UV
irradiation. The invention concerns the general curing of UV curing
printing inks independent of the particular fixing agent
system.
[0004] Conventional applications for UV curing printing inks are
e.g.: sheet-fed offset printing (e.g. packaging), continuous offset
printing (e.g. direct mail advertising), dry offset printing
(indirect letterpress printing, e.g. cups and tubes), label
printing (letterpress and flexographic printing), flexographic
printing (e.g. packaging) and silk screen printing (e.g. technical
articles). UV curing, also often referred to as UV drying, has the
advantage that the printing inks are solvent-free or of low solvent
content and cure rapidly on the printed material under UV
irradiation so that the printed material can be promptly further
processed or packaged. The invention concerns curing of the printed
ink and is therefore independent of the particular printing process
used for introducing the printing ink onto the printed
material.
[0005] Substantial technical requirements are required for the
industrial radiation curing of printing ink. Prior art has required
very high output power for the UV radiation sources in order to
satisfy demands for ever increasing production speeds of 100 to 400
m/min and higher. In multi-color printing, the separation between
printing devices must be kept small to guarantee the precise
matching of sequential colors without excessive complication and
expense. The maximum separations in combination with the high
printing speeds lead to extremely short times within which the ink
must be sufficiently cured to prevent smearing during subsequent
handling. Practical separations between printing devices assume
values of circa 0.3 to 1.0 m corresponding to production times
between printing stations of about 0.1 sec.
[0006] When one considers these stringent requirements it becomes
clear that the UV intensity of the radiation source is very
important. In order to achieve this, mercury vapor high pressure
and medium pressure lamps have been nearly exclusively used as UV
radiation sources in practical industrial applications up to this
point in time. These lamps facilitate a particularly high UV
intensity. DE-3902643 C2 and DE 4301718 A1 provide examples
therefor.
[0007] The arc lengths of the conventional lamps vary between 10 cm
and 220 cm and the specific electric power lies in the range
between 30 to 250 watts per centimeter of arc length. The UV light
power assumes values of approximately 20 watts per centimeter of
arc length. Due to the need for UV transparency, the tubular lamp
material is quartz and the lamps are operated with a gas pressure
of 1 to 2 atm. In certain cases, lasers, in particular excimer
lasers, are also used to produce the UV radiation.
[0008] The above mentioned conventional UV radiation sources have
the advantage of being able to produce a very high UV intensity on
the surface of the printed material to effect very short curing
times in the range of tenths of seconds. Excimer lasers have the
disadvantage of being complicated and expensive. For this reason,
medium and high pressure gas discharge lamps are more widely used.
They have, however, the disadvantage that their efficiency of UV
light production in the relevant spectral region is only 20%, so
that 80% of the introduced energy is dissipative power and must be
removed by cooling.
[0009] Due to the high power consumption and high dissipative power
of the lamps, their surface temperatures are in the range of 800 to
900.degree. C. which necessitates special measures for cooling
their surroundings. Since the lamps cannot be immediately restarted
after having been switched-off, one must also provide means for
preventing the printing ink introduced onto the printed material or
the printed material itself from burning when the printing machine
is in paused operation. Heat protection glass, which is sometimes
cooled, and pivoting reflectors are therefore provided. The power
consumption portion of the drying device used in a conventional
printing machine having an overall power consumption of 100 kW,
assumes values in excess of 50 kW and typically 80 kW.
[0010] Conventional use of medium pressure and high pressure lamps
is therefore very complicated and expensive and is associated with
high power consumption. The associated disadvantages have, however,
been accepted in the art of radiation curing printing technology,
since one has assumed up to this point in time that very high
intensity UV lamps having high UV radiation power were necessary to
achieve shorter curing times.
[0011] The publication Industrie-Lackier-Betrieb [Industrial
Coating and Painting], 1969, pages 85-91 proposes the use of
so-called actinic or super-actinic fluorescent lamps to reduce
thermal loads when curing UV curable coatings. These are special
low pressure lamps having a fluorescent coating which shifts the
intensity maximum towards the red to achieve a spectrum having high
fractions in the UV-A region. The high UV-A fractions have been
considered necessary by those of average skill in the art in order
to achieve rapid reaction times. Those skilled in the art of curing
pigmented systems such as printing inks were of the same
opinion.
[0012] JP 59189340 A2 (Derwent reference No. 84-303796/49) proposes
a compound for use as printing ink which can be cured by a
plurality of different UV radiation sources, including high
pressure, medium pressure, and low pressure mercury lamps. The
applications described in this publication suggest that the lamps
primarily emit in the UV-A or visible spectral region and that
relatively long irradiation times, not compatible with rapid
industrial production processes, were required.
[0013] Departing from this prior art, it is the underlying purpose
of the invention to create a process and an associated device for
curing a UV curable printing ink on a printed material which avoids
the disadvantages of conventional UV gas discharge lamps associated
with their high heat production.
[0014] In order to achieve this purpose, it is proposed in the
above mentioned process and corresponding device to use a low
pressure gas discharge lamp as the UV radiation source which has an
integrated spectral radiation flux in the UV-B and UV-C region in
excess of 50%, preferentially in excess of 75% of the UV radiation
flux.
[0015] In accordance with the present invention, one has
surprisingly discovered that the extremely stringent requirements
for radiation curing of printing inks can be satisfied by low
pressure gas discharge lamps without--as had been previously
considered necessary--having their wavelength spectrum somewhat
shifted or substantially shifted towards longer wavelengths.
[0016] The range of the UV spectrum as well as its subdivision into
various regions are not consistently defined in the literature.
Within the framework of the invention, the UV spectrum is
subdivided in accordance with DIN 5031, Part 7. It includes the
region between 100 and 380 nm, wherein the UV-C range extends from
100 to 280 nm, the UV-B region from 280 to 315 nm and the UV-A
region ranges from 315 to 380 nm. Spectral radiation flux refers to
the radiation power in watts per nm as a function of wavelength.
The radiation flux is a measure of the intensity of the radiation.
Integration or summation of the spectral radiation flux over a
wavelength interval gives the radiation flux irradiated within this
wavelength interval.
[0017] Low pressure gas discharge lamps in accordance with the
invention are lamps which can normally be operated with gas
pressures between 10 mbar and 50 mbar, preferentially between 20
mbar and 30 mbar. Their specific electric power consumption is
substantially less than that of medium and high pressure lamps and
lies in the range of 0.2 to 2.5, preferentially between 0.5 and 1.0
watts per cm of arc length. Although the low pressure gas discharge
lamp efficiency for the relevant UV region is higher than that of
conventional lamps and amounts to 30 to 40%, their overall UV
radiation flux is substantially less than that of conventional
lamps. It assumes values of circa 0.2 watts per centimeter of arc
length and is therefore about a factor of 100 less than that of
previously used conventional medium and high pressure lamps.
[0018] It has surprisingly turned out that UV curing printing inks
can also be satisfactorily cured using low pressure gas discharge
lamps even when the printing ink is irradiated with a UV intensity
of radiation between 1 and 100 mW/cm.sup.2, preferentially between
10 and 50 mW/cm.sup.2. The UV intensity of radiation of medium and
high pressure lamps on the printed material is approximately 1
W/cm.sup.2. This radiation intensity on the printed material refers
to the radiation flux per unit area incident on the printed
material. The printed material can be tilted at an angle with
respect to the direction of the radiation. The intensity of
radiation has the units of W/cm.sup.2.
[0019] The use of low pressure gas discharge lamps in accordance
with the invention has various significant advantages in practical
applications. Their surface temperature is substantially lower.
Mercury vapor lamps have temperatures during normal and optimized
operation of about 30.degree. C. Amalgam lamps, which have the
advantage compared to mercury discharge lamps of having a somewhat
higher UV light yield, have temperatures during normal operation of
circa 120.degree. C. These lower surface temperatures, in
combination with the reduced power consumption, lead to a
substantially reduced temperature loading of the surroundings of
the lamp and of the printed material.
[0020] The reduced heating of the counter-pressure cylinder also
has technical advantages, in particular for multi-color printing
devices. Up to this point in time, a high degree of complication
and expense was necessary to keep the counter-pressure cylinder at
a constant temperature. This was of central importance for the
quality and execution of the printing process. The reduced
temperature loading allows for the printing of materials with UV
curable printing ink which could not previously be printed. An
example is temperature-sensitive plastic foils (e.g. heat shrinking
foils).
[0021] The relatively low power requirements of low pressure gas
discharge lamps and the technical simplicity of associated cooling
allows for the reduction of the fraction of power consumption for
the drying unit: in a printing machine having an overall power
consumption of 100 kW, to about 10 to 15 kW or less. The power
consumption of a medium pressure gas discharge lamp with associated
cooling fan assumes a typical value of about 3.5 kW. In contrast
thereto, the power consumption of 10 low pressure gas discharge
lamps in accordance with the invention, including associated fans,
is only approximately 400 W.
[0022] In addition to the reduced heat load, the reduced danger of
burning, and the reduced power loss, the low pressure gas discharge
lamps have the added advantage of being quickly exchangeable. The
lamps require nearly no cooling-down time following failure and can
therefore be more rapidly replaced. Furthermore, low pressure gas
discharge lamps have the additional advantage compared to
conventional lamps of requiring little or no warm-up time before
reaching stable operating conditions. They can also be restarted
immediately after being switched-off and have an intensity which
can be regulated. In addition, in contrast to medium pressure
lamps, there is no danger that drops of ink or dirt particles burn
into the bulb to destroy the lamp. The lifetime of low pressure gas
discharge lamps is about 8000 hours, which is at least four times
that of medium pressure lamps.
[0023] In addition, the amount of ozone produced through operation
of low pressure gas discharge lamps is substantially less than that
of medium pressure lamps. This is due to the fact that low pressure
gas discharge lamps emit little or no radiation at the critical
wavelength of 185 nm at which ozone is produced in atmospheric
oxygen. In contrast thereto, medium pressure lamps cause
substantial ozone production.
[0024] In summary, the invention achieves goals which have been
sought by those of average skill in the art for a long time. The
following measures are preferentially used individually or in
combination to assure particularly good results with regard to
quality and speed of the configuration as well as with regard to
the structural requirements of the printing machine.
[0025] It can be advantageous when the integrated UV-B spectral
radiation flux is in excess of 50%, preferentially more than 75% of
the UV radiation flux. In this case, the lamps are designated UV-B
lamps. It can also be advantageous if the UV-C integrated spectral
radiation flux is in excess of 50%, preferentially more than 75% of
the UV radiation flux. In this case, the lamps are designated UV-C
lamps.
[0026] Within the framework of the invention, both UV-C as well as
UV-B low pressure gas discharge lamps have turned out to be
advantageous for the curing process. With UV-C low pressure gas
discharge lamps, the radiation flux integrated over the UV-C range
can assume values in excess of 50%, preferentially more than 75% of
the UV radiation flux. In the case of a UV-B low pressure gas
discharge lamp, the corresponding UV-B integrated spectral
radiation flux can assume values in excess of 50%, preferentially
more than 75% of the UV radiation flux.
[0027] The maximum of the spectral radiation flux distribution of
the low pressure gas discharge lamps, in particular of the UV
radiation flux, can advantageously lie in the UV-B or UV-C region.
For a line spectrum, this refers to the wavelength having the
highest UV intensity. For a continuous spectrum, this requirement
refers to the maximum of the spectral radiation distribution. When
the UV spectrum has both lines and continua, this feature refers to
the maximum with regard to the line and continuous emission
regions.
[0028] An additional advantageous feature proposes use of a low
pressure gas discharge lamp having an integral spectral UV
radiation flux above a wavelength of 190 nm, in particular above
240 nm, which is more than 50%, preferentially more than 75% of its
UV radiation flux, in particular of its UV-C radiation flux. It is
particularly advantageous when the integrated UV-C radiation flux
above a wavelength of 190 nm, in particular above 240 nm, is more
than 50% and preferentially more than 75% of the UV radiation
flux.
[0029] It is particularly advantageous when the low pressure gas
discharge lamp emits more than 50%, preferentially more than 75% of
the radiation flux of its UV light in the UVC region above a
wavelength of 240 nm. The lamp then differs substantially from the
medium pressure lamps having their main emitted UV spectral
fraction in the UVB or UV-A region. Since not only the overall
intensity but also the distribution of the individual lines can be
of significance, it is advantageous when the above mentioned
conditions apply to wavelengths having an intensity of more than
20% of the UV wavelength of highest intensity. The intensity
maximum of UV-C low pressure gas discharge lamps is normally in the
wavelength range between 249 and 259 nm, in particular at 254
nm.
[0030] The likewise advantageous UV-B low pressure gas discharge
lamps are also designated as UB-B fluorescent lamps. They have a
phosphor coating which shifts the maximum of the radiation flux
into the UV-B region. The maximum preferentially lies above 305 nm.
The actual positions of the intensity maximum and of the emitted
lines as well as, in particular, their linewidths can be influenced
by the phosphor or the phosphor mixture. The possible band widths
thereby range from very narrow, nearly monochromatic UV-B radiation
up to emissions which nearly span the entire UV-B range. The UV-B
low pressure gas discharge lamps advantageously emit in excess of
50%, preferentially more than 75% of their UV light in the UV-B
region.
[0031] Within the framework of the invention, low pressure gas
discharge lamps are generally preferred whose emission spectra are
not shifted towards longer wavelengths through the addition of
fluorescent materials. This means that neither an actinic nor a
super-actinic gas discharge lamp is used. The UV-B lamps are not
quite as advantageous as the UV-C lamps, since their light output
is lower due to the light conversion step and since the printing
ink can be less reactive in the spectral region in which they emit
than in the UV-C region. They nevertheless likewise constitute an
economically interesting improvement over conventional medium
pressure and high pressure lamps.
[0032] In accordance with an additional advantageous feature, a
plurality of low pressure gas discharge lamps of differing emission
spectra can be utilized, in particular a combination of a UV-C and
a UV-B low pressure gas discharge lamp.
[0033] The advantageous use of low pressure gas discharge lamps
having differing emission spectra to produce mixed light can be
effected not only by having differing lamps, but also using lamps
which are partially coated with fluorescent material. The ratio of
integrated UV-B to integrated UV-C radiation flux can lie between
0:1 and 1:0. A higher UV-C fraction is however normally preferred
for the above mentioned reasons.
[0034] The fixing agents of conventional UV cured printing inks are
normally tuned to the particular radiation of the UV radiation
source. One would therefore expect that conventional printing inks
are not suitable for the invention and that special fixing agent
systems or, in particular, special photo-initiators would be
necessary which are tuned to the UV spectrum of the low pressure
gas discharge lamps used in accordance with the invention. It is
clearly possible for one of average skill in the art to develop
printing ink compositions and photo-initiators which are optimized
and specially tuned to low pressure gas discharge lamps. It has
however surprisingly been discovered within the framework of the
invention that good curing results can also be achieved using
conventional printing inks. This is particularly true for e.g. the
XKC-Series UV-Flex inks of Gebruder Schmidt Druckfarben, Frankfurt,
in particular of type 80 XKC 1004-1.
[0035] A printing ink is, by way of example, suitable for the
process in accordance with the invention which has a fixing agent
system containing the following components: a) one or a plurality
of cycloaliphatic epoxy resins as curable fixing agent and b) one
or a plurality of arylsulfonium salts as photo-initiators. A
cycloaliphatic epoxy resin is a cationic curable fixing agent.
Clearly, the ink can also contain additional conventional
components such as additional photo-initiators, solvents, pigments,
dyes, thinning agents, reactive thinner, wax, leveling agent,
wetting agents or other additives.
[0036] In accordance with an additional advantageous feature,
component b) contains a triarylsulfonium salt. It is thereby
preferred when the triarylsulfonium salt contains a
triarylsulfoniumantimonate, in particular a
triarylsulfoniumhexaflouroantimonate. One can advantageously
further provide that the component b) contains a mixture of
different arylsulfonium salts. The printing ink can also contain
other fixing agents in addition to the cycloaliphatic epoxy
resin.
[0037] Printing technology primarily uses radical curable printing
inks, since these provide, when irradiated with conventional medium
pressure lamps, a drying time which is shorter than that of a
cationic curable printing ink. The radical curable inks have the
additional advantage that their chemical composition can be widely
varied. However, the most prevalent fixing agents absorb mostly in
the UV-C range. As a result, only a small amount of printing ink
reactivity can be effected even with photo-initiators absorbing in
the UV-C region. In contrast thereto, the fixing agents used with
cationic curable printing inks are substantially transparent in the
UV-C region. A high degree of reactivity can therefore be achieved
even with a UV-C or UV-B low pressure gas discharge lamp. Cationic
curable inks based on epoxy resins are preferred within the context
of the invention for the above mentioned reasons. Radical curable
inks can however also be used.
[0038] It is generally advantageous to cure a printing ink with the
printing process in accordance with the invention which has fixing
agent components which are substantially transparent in the UV-C or
UV-B regions of the UV-light from the low pressure gas discharge
lamp. In this manner, deeper layers can also be reached by adequate
amounts of UV light. This means that the absorption curve of the
fixing agent should be shifted towards shorter wavelengths compared
to standard fixing agents used in medium and high pressure lamps.
Offset printing is associated with typical layer thickness of 1 to
3 .mu.m and flexographic printing with layer thickness between 3
and 8 .mu.m. In addition, the squeezed edges have a thickness of at
most 20 .mu.m. The fixing agent should therefore be sufficiently
transparent up to a thickness of 20 .mu.m. This implies that the
transparency of the fixing agent is preferentially sufficiently
high up to this layer thickness that it does not absorb more than
half of the incident UV intensity of the low pressure gas discharge
lamp. Accordingly the system of fixing agent and photo initiator is
such that preferentially more than 10% of the UV light is absorbed
up to a layer thickness of 20 .mu.m.
[0039] The properties of the fixing agent, in particular its
transparency to the UV light used and the reactivity of the fixing
agent and photo-initiator system are of particular importance for
the use of a printing ink within the context of the invention. In
addition, as is usual, the individual components should be mixable
and mutually compatible so that no spontaneous reactions are
triggered. Filling agents and additives can be used in liquid or
solid form and are subject to the same requirements with regard to
UV light transparency as is the fixing agent.
[0040] The pigments can be organic or inorganic in nature.
Inorganic compounds are generally solids and organic compounds can
be solid or liquid. The concentrations and absorption properties of
liquid pigments should be appropriately adjusted. This is also true
for solid pigments with which additional grain-size dependent
scattering effects can occur.
[0041] The printing ink should be sufficiently reactive to the UV
light and capable of activation by same. This is particularly true
for the photo-initiators which should be sufficiently reactive in
the wavelength range used. The reactivity has two aspects. First of
all, the UV light absorption must be sufficiently large. In
addition, the photo-initiators should also properly transfer or
feed the absorbed energy into the corresponding radicals (radical
polymerization) or acids (cationic polymerization) to initiate the
chain reaction for polymerization. The photo-initiator should
therefore be present in suitable concentrations and be sufficiently
absorbing. It must also be capable of transferring the absorbed UV
light energy to the monomers for both radical as well as cationic
curing.
[0042] It is also possible to use a plurality of photo-initiators
in one printing ink having different absorption properties. The
chain reaction initiators then differ from the light absorption
activators.
[0043] It is generally advantageous to cure a printing ink having
fixing agents largely transparent to the UV light emitted by the
low pressure gas discharge lamp with photo-initiator components
which highly absorb the UV light emitted by the low pressure gas
discharge lamp, which are reactive and which can also be activated
in this wavelength region. It is therefore generally advantageous
for the fixing agent and photo-initiator components of the printing
ink to be composed and adapted to each other in such a manner that
the printing ink can be cured up to a layer thickness of 20 .mu.m
using the UV light emitted by the low pressure gas discharge
lamp.
[0044] It is furthermore preferred when the printing ink has a high
reactivity even at room temperature. In the process in accordance
with the invention, the printing ink is heated only slightly or not
at all. The temperature during UV curing is preferentially not
higher than 40.degree. C. Conventional high and medium pressure
lamps have substantially higher temperatures and associated
application related disadvantages.
[0045] In accordance with an advantageous feature, the time
duration of the UV irradiation curing of the printing ink is less
than 2 seconds, preferentially less than 1 second. The short
printing ink reaction time advantageously allows for the
realization of high production speeds or smaller separations
between the individual printing stations. The reaction time is
thereby that time which passes until the surface of the printing
ink is no longer sticky so that the printed material can be printed
in additional printing stations or otherwise processed. The curing
time can be substantially longer. With radical curing printing
inks, the curing time is not substantially longer than the reaction
time. For cationic curing printing inks, the UV irradiation
normally only initiates the process, i.e. pre-cures. Subsequent
complete curing can be rapid or could also take up to 24 hours. As
mentioned, the short irradiation time or reaction time is not only
of significance for the printing of an increased number of objects
per unit time, rather also in multi-color printing. The passer
problem associated therewith necessitates small separations between
printing stations and associated rapid intermediate drying to
prevent spreading of ink.
[0046] The amount of time during which the printing ink is
irradiated with UV light depends on the speed with which the
printed material and associated printing ink move relative to the
low pressure gas discharge lamps for UV curing as well as on the
area irradiated by the low pressure gas discharge lamps. With the
method in accordance with the invention, printing processes can be
advantageously carried out with which the printed material moves
with a path velocity of more than 20 m/min., of preferentially more
than 40 m/min., and particularly preferably of more than 50
m/min.
[0047] The use of low pressure gas discharge lamps in accordance
with the invention, in particular in combination with the above
described UV curing printing inks, has turned out to be
particularly advantageous in flexographic printing. This is true
for all flexographic printing machine concepts which can be
classified as follows:
[0048] 1. The multi-cylinder printing machine is a rotating type
having four or six individual printing devices associated with one
station, in particular for printing a plurality of colors.
[0049] 2. The tandem printing machine is a rotating type with each
printing device disposed in its own individual station.
[0050] 3. The one cylinder or central cylinder machine is a
rotating type having the printing device disposed about one common
central counter-pressure cylinder.
[0051] The device in accordance with the invention for curing a UV
curing printing ink on a printed material by means of which the
printing ink is irradiated with UV light from a UV light source, in
particular for carrying out a process in accordance with the
invention, is characterized in that the UV radiation source
comprises a low pressure gas discharge lamp whose integrated UV-B
and UV-C spectral radiation flux is in excess of 50%,
preferentially more than 75% of the UV radiation flux. A device of
this kind is subsequently designated with the conventional name
"drier".
[0052] Depending on the application, the drier can include one or
more UV radiation sources. If a plurality of UV radiation sources
are used, these could be the same or of differing types. It can
also be advantageous in certain special cases to provide for
conventional radiation sources in addition to the low pressure gas
discharge lamp. The exclusive use of low pressure gas discharge
lamps is preferred. An advantageous feature proposes that a drier
comprises more than four, preferentially more than eight, low
pressure gas discharge lamps.
[0053] An advantageous embodiment can be particularly characterized
by the fact that the drier comprises a plurality of adjacently
disposed low pressure gas discharge lamps. In this fashion, a high
UV radiation intensity per unit area on the printed material or a
smoother spatial illumination can be effected. Alternatively, a
relatively large area can thereby be illuminated. The low pressure
gas discharge lamps can be bar-shaped. However, it is preferred
when the drier comprises a plurality of U-shaped low pressure gas
discharge lamps disposed next to each other at the longitudinal
sides of the U-shape. U-shaped low pressure gas discharge lamps
have the advantage of effecting a relatively high illumination
intensity. The low pressure gas discharge lamps can be arranged in
particularly close proximity to another if they are disposed in
alternately opposite directions. The open and closed ends of the
U-shape form an alternating series and the open ends having
electrical contacts can be connected to electrical contact elements
without having the separation between the low pressure gas
discharge lamps be limited by these contact elements.
[0054] The separation between the lamps and between the lamps and
the printed material is preferentially subject to the requirement
that the radiation intensity in the plane of the printed material
within the principal effective region, i.e. excluding e.g. the
entrance and exit zones, is as homogeneous as possible. If the
printed object is moved along a transport direction and/or rotated
in the curing zone, this condition applies to the time-integrated
intensity during passage through the drier.
[0055] The low pressure gas discharge lamps can be disposed at
close relative separations to effect a compact assembly and/or to
realize a homogenous irradiation intensity which deviates by less
than 30%, preferentially by less than 20% from an average value.
The low pressure gas discharge lamp bulbs can even touch without
mutual separation. The separation between the bulbs of the low
pressure gas discharge lamps preferentially does not exceed 30%,
preferentially not more than 20% of the low pressure gas discharge
lamp bulb diameter.
[0056] The separation between the low pressure gas discharge lamps
and the printed object should be sufficiently large to prevent
contact with the lamp in case that the position of the printed
object should be subject to variations. A reasonable practical
minimal separation is 1 cm. The upper limit of the separation
between the surfaces of the low pressure gas discharge lamps and
the printed material can advantageously be less than 5 cm.
[0057] It is furthermore preferred when the device comprises a
reflector for reflecting the UV light emitted by the low pressure
gas discharge lamps onto the curing printing ink. The reflector can
be used to reflect UV light for UV curing which is not emitted by
the low pressure gas discharge lamps in a direction towards the
printed material as well as to effect a more even illumination of
the printed material. If the printed materials are extended, the
reflector is preferentially disposed on that side of the low
pressure gas discharge lamps which faces away from the printed
material to reflect the UV light emitted by the low pressure gas
discharge lamps in a direction towards the printed material. If the
printed material is not an extended flat object, it can also be
advantageous to dispose a reflector at that side of the printed
material facing away from the irradiation source to more evenly
illuminate the printed material at all sides.
[0058] Reflectors disposed at that side of the low pressure gas
discharge lamps facing away from the printed material are also
known in the art for use with medium and high pressure lamps. They
normally comprise metal plates and can be pivoted to reduce the
heat load on the printed material during pauses in the operation of
the installation. A reflector in accordance with the invention can,
in contrast thereto, preferentially be stationary. The heat loading
of the printed material caused by the low pressure gas discharge
lamps is not severe. Since the lamps can be immediately restarted,
they can also be switched-off if necessary. The reflector is
therefore less complicated and less expensive.
[0059] The reflecting layer of the reflector can be assembled from
planar portions. In a preferred configuration which is particularly
easy to manufacture, the reflector comprises one single planar
reflecting layer. If the reflector is also stationary, the
configuration is particularly simple to realize.
[0060] Improved optics can be achieved in other advantageous
configurations in which the reflecting layer of the reflector has
concave portions curved with respect to the low pressure gas
discharge lamp. In addition, the reflector can be arranged in a
conventional manner at a separation from the low pressure gas
discharge lamp. The reduced surface temperature of the low pressure
gas discharge lamps also allows for the reflector to be in line or
surface contact with the low pressure gas discharge lamp. In this
manner, a very compact construction of the drier is effected which
nevertheless has increased light output. Surface contact can
advantageously occur over 30% to 60% of the surface of the bulb or
of the periphery of a cross section through the low pressure gas
discharge lamp respectively. The optimal values for each case
depend on the size of the printed object and its separation from
the low pressure gas discharge lamp.
[0061] It can be advantageous when the reflector comprises a
dielectric mirrored layer to achieve a high degree of reflection. A
dielectric mirrored layer is a multi-layered system of optical
coatings for increasing the amount of reflection. The reflector
itself can thereby be fashioned from metal, glass or another
suitable material.
[0062] The reflector is preferentially diffuse reflecting in order
to achieve a more even spatial irradiation intensity on the printed
material and i.e. comprises a reflecting layer made from optically
diffuse reflecting material. Optically diffuse reflecting materials
are materials which, due to their composition, diffusely reflect
incident optical radiation or diffusely pass penetrating radiation.
They can therefore be designated as Lambert surfaces or Lambert
radiators. They are usually mat white.
[0063] The optically diffuse reflecting material can be made from
conventional ceramic plate or from metallic reflectors having a
roughened, metallic reflecting surface (e.g. aluminum plates). A
coating can be used comprising, in particular, a transparent
material containing diffuse reflecting particles such as barium
sulfate, titanium oxide or magnesium oxide.
[0064] A particularly advantageous feature proposes that the
optically diffuse reflecting material of the reflector reflecting
layer comprises a matrix made from a transparent matrix material
consisting essentially of a curable silicone rubber with imbedded
reflecting particles. A material of this kind is optically,
chemically, biologically, and thermally resistant, is insensitive
to soiling and can also be easily cleaned. It has a good resistance
to aging and has high transparency, in particular to UV.
[0065] The matrix material in accordance with the invention
consists essentially of silicone rubber. By essentially is meant
that the silicone rubber does not contain any amount of foreign
materials which would be intolerable for obtaining the desired
properties, so that the properties of the matrix material are
determined by the silicone rubber. As a rule, the matrix material
normally consists of silicone rubber having a standard commercial
or preferentially higher purity of e.g. in excess of 95%.
[0066] In principle, all conventional silicone rubber is usable
within the framework of the invention. A suitable silicone rubber
material can be selected which has the necessary matrix material
properties in dependence on the application. Both condensation
cross-linked as well as addition cross-linked rubber may be
used.
[0067] Silicone rubbers can be advantageously poured to facilitate
inexpensive creation of arbitrary shapes for various applications.
Other economical production processes, such as extrusion, are
advantageous and possible. The thin liquid curable silicone rubber
is initially processed and vulcanization subsequently occurs to
form a cured, solid matrix material. For most applications, it is
advantageous when the Shore A hardness of the cured matrix material
assumes values in accordance with the DIN standard 53505 of between
20 and 90. The matrix material has an advantageous intrinsic
solidity in this range.
[0068] In accordance with an advantageous feature, the reflecting
particles are present within the matrix material in powdered form.
For most applications, the reflecting particles should be
homogeneously imbedded in the matrix material. For special
applications, it can however be advantageous for the concentration
of reflecting particles in the matrix material to increase or
decrease with depth.
[0069] All conventional diffuse reflecting substances are suitable
for use as diffuse reflecting particles in accordance with the
invention. Examples of such diffuse reflecting substances are
magnesium oxide, aluminum oxide, titanium dioxide,
polytetrafluoroethylene (Teflon (.sup.R) or silicon dioxide
(Aerosil (.sup.R). Barium sulfate has turned out to be particularly
advantageous within the context of the invention.
[0070] The diffuse reflecting particles substantially include one
or more of the above mentioned substances. By substantially is
meant that other particles are not present in the material or are
only present to such an extent that, for the actual application,
the diffuse reflection properties are determined by the particles
to satisfy the particular requirements in each case. The particles
are normally contained within the matrix material as pure
substances having a high commercially producible purity, e.g. in
excess of 99%. A high purity and homogeneous distribution can be
particularly advantageous in optics applications. The particles of
each substance can have one grain size or comprise a mixture of
differing grain sizes to achieve special spectral properties.
[0071] The reflecting particles in the material in accordance with
the invention can comprise only one of the above mentioned
substances or can be a mixture of two or more differing substances.
For production related technical reasons, an admixture of particles
of only one substance is preferred. In special applications, in
particular for effecting a specific spectral dependence, it can
however be advantageous to utilize a mixture of differing
substances and/or a mixture of differing grain sizes.
[0072] The grain size of the above mentioned particles
advantageously lies substantially between 1 .mu.m and 100 .mu.m:
for the case of silicon dioxide (Aerosil (.sup.R), between 10 nm
and 200 nm. Substantially thereby means that the average value of
the grain size distribution lies in this range. Since the grain
size of particles or of powder has a certain tolerance or grain
size distribution in dependence on production processes, a small
amount e.g. up to 5% of the particles can also be present which lie
outside of the above mentioned size range.
[0073] The full width half maximum of the actual grain size
distribution can be critical in certain applications and rather
insignificant for other applications. Trial and error can determine
which particle sizes and particle size distributions produce the
desired reflection properties for the actual case at hand.
[0074] The material in accordance with the invention has the
advantage of having a wide range of applicability. It can be easily
manufactured and tailored mechanically and optically to the case at
hand. It can be self-supporting and of nearly arbitrary shape or be
securely disposed on a substrate, wherein it can level and cover
unevenness in the substrate. It is optically, thermally and
biologically stable and temperature-insensitive. It can be easily
cleaned and absorbs little light. The actual properties can be
optimized for the particular application requirements. In addition,
it is easily and therefore inexpensively processed. The hardness
can be adjusted within a wide range to facilitate many differing
applications. For example, flexible mats of suitable stability can
be produced e.g. by molding which have arbitrary curved and bent
shapes. The material can be easily worked and mechanically
processed. It can be solid or flexible and can also be glued.
Shaped objects can be molded or produced by injection molding. The
material has no intrinsic color and therefore does not
disadvantageously influence the spectrum. The surface of the
reflecting material facing the emitted light must not have the mat
finish required with conventional materials. It must not have a
"molecular roughness" in order to effect good diffuse reflection
performance. For this reason, it can be molded in a mold having a
smooth surface.
[0075] The material in accordance with the invention is
preferentially produced by mixing of the particles into a liquid
matrix material under vacuum. In this manner, a vulcanized material
can be produced without bubbles.
[0076] Further advantageous features and highlights can be
recognized by means of the following embodiments of the invention
and are described in further detail below with reference to the
schematic representation of the drawings.
[0077] FIG. 1 shows a schematic cross section through a drier
according to prior art in an operating state,
[0078] FIG. 2 shows a schematic cross section through a drier
according to prior art in a paused condition,
[0079] FIG. 3 shows a modification of FIG. 1,
[0080] FIG. 4 shows a modification of FIG. 2,
[0081] FIG. 5 shows a schematic cross section through a first drier
in accordance with the invention,
[0082] FIG. 6 shows a first modification of FIG. 5,
[0083] FIG. 7 shows a second modification of FIG. 5,
[0084] FIG. 8 shows a perspective view of FIG. 5,
[0085] FIG. 9 shows a perspective view of FIG. 6,
[0086] FIG. 10 shows a perspective view of FIG. 7,
[0087] FIG. 11 shows a modification of FIG. 8,
[0088] FIG. 12 shows a modification of FIG. 9,
[0089] FIG. 13 shows a modification of FIG. 10,
[0090] FIG. 14 shows a schematic view of a plurality of lamps,
[0091] FIG. 15 shows a modification of FIG. 14,
[0092] FIG. 16 shows a schematic cross section through a drier and
a printing machine,
[0093] FIG. 17 shows a schematic cross section through a drier in
accordance with the invention,
[0094] FIG. 18 shows a detail of FIG. 17,
[0095] FIG. 19 shows a relative spectral radiation flux of a high
pressure mercury vapor lamp,
[0096] FIG. 20 shows a relative spectral radiation flux of a low
pressure gas discharge mercury vapor lamp, and
[0097] FIG. 21 shows a spectral radiation flux of a UV-B low
pressure gas discharge lamp.
[0098] FIG. 1 shows a schematic cross section through a drier 20
according to prior art in its operating state, with curing printed
material 9 which has been printed with UV curing printing ink 14
passing thereby. A medium pressure gas discharge lamp, UV radiation
source 8, produces UV light to trigger polymerization of the
printing ink 14. The printed material 9 is fed past the UV
radiation source 8 in transport direction 10. Pivoting reflectors
21 are provided for smoothing out the illumination intensity on the
printing ink 14 and for increasing the light yield. They can each
be pivoted via a turning device 11 from the operating condition
shown in FIG. 1 into the paused position shown in FIG. 2. The
reflectors 21 must be pivoted, since the medium pressure lamp has a
very high surface temperature and would burn the printed material 9
when it is stationary relative to the lamp 8.
[0099] Heat protection glass 22 is disposed between the UV
radiation source 8 and the printed material 9 to protect the
printed material 9 and the printing ink 14 from the high amount of
heat emanating from the medium pressure lamp. FIGS. 3 and 4 are
modifications of FIGS. 1 and 2 having cooling pipes 35 flown
through by water instead of the heat protection glass 22 to remove
the heat.
[0100] FIG. 5 shows a schematic cross section through a drier 20 in
accordance with the invention. In this case, the UV radiation
sources 8 comprise a plurality of mutually adjacent low pressure
gas discharge lamps 7 which are passed by the printed material 9,
printed with printing ink 14, which is fed in transport direction
10 to cure the printing ink 14. Advantageous low pressure gas
discharge lamps are, in particular, the UV-C lamps of type TUV
produced by Philips having a principal emission at 254 nm and the
UV-B lamps of type TL/01 with principal emission at 311 to 312 nm
or of type TL/12 having principal emission at 306 nm. They have a
high efficiency for UV light and can be operated with nearly no
ozone production. The low heat output of the low pressure gas
discharge lamps permits the reflector 5 to be stationary and at a
small separation from the lamps. The separation between the
reflector 5 and the UV radiation sources 8 can be less than twice
the diameter, preferentially less than one time the diameter, of
the bulb 16 of the UV radiation sources 8.
[0101] In the embodiment shown, the reflector 5 comprises three
planar reflectors 5. One large reflector 5a is disposed on that
side of the lamps 8 facing away from the printed material 9. Two
smaller reflectors 5b are disposed at the sides. The reflectors
comprise a reflecting layer made from a reflecting material 1. The
reflecting material 1 can e.g. be a conventional ceramic plate or
metallic reflector. The metallic reflector can have a roughened,
metallic reflecting surface and be made e.g. from aluminum.
[0102] It is preferred when the reflecting layer of the reflector 5
consists essentially of an optically diffuse reflecting material in
accordance with the invention having a matrix material made from
cured silicone rubber with a homogeneous distribution of particles
imbedded therein.
[0103] These particles comprise powdered barium sulfate having a
grain size of about 50 .mu.m. The particles are not visible in FIG.
5 due to their small size. The ratio of particles to matrix
material is approximately 1:10 by weight. A ratio smaller than
1:100 does not normally result in sufficiently high reflectivity.
Weight ratios in excess of 1:1 normally result in a degree of
filling of the matrix material by the particles which is so high
that the silicone becomes brittle or does not properly
vulcanize.
[0104] The reflector 5 has a reflectivity in excess of 90%. The
reflecting layer of material 1 has a thickness of several
millimeters. It may advantageously lie in the range between 0.1 and
10 mm. The reflector 5 can therefore be a so-called volume
reflector. Such a reflector differs from a purely surface reflector
in that reflection also occurs from deeper material layers.
[0105] The reflecting material 1 or the reflecting sheet metal is
disposed on a substrate 6 or on a portion of the housing 27. The
matrix material 2 or the reflecting material 1 can be a
condensation cross-linked silicone rubber directly bound to the
supporting surface and can, for example, be extruded onto the
substrate 6. If the matrix material is an additive cross-linked
rubber, a suitable bonding process, e.g. gluing, can be used to
bind it to the supporting surface.
[0106] Advantageous silicone rubbers are in particular those sold
by the company Wacker-Chemie GmbH [Wacker Chemical Incorporated],
Munich under the name "Elastosil (R)", in particular the types M
4600, R 401, R 402, R 411, R 420, R 4000, and R 4105 as well as
Semicosil, in particular the types 911, 912, and types RTV-E 604,
RTV-ME 601, and SilGel 612.
[0107] FIG. 6 shows a modified reflector 5. The reflecting material
1 layer consists essentially of a matrix material in accordance
with the invention made from silicone rubber having diffuse
reflecting particles. It is mounted to a substrate 6 or to a
housing portion 27.
[0108] The reflecting layer is characterized in that its surface
facing the radiation sources 8 has curved concave portions with
respect to the radiation sources. They are disposed at a small
separation from the surface of the bulb 16 of the corresponding UV
radiation source 8. This separation can be less than one half of
the diameter of the UV radiation source 8 bulb 16. Towards this
end, the center of each curve of the reflector 5 can lie inside the
associated low pressure gas discharge lamp, in particular at its
center. In this manner a compact configuration can be realized
which provides for a homogeneous illumination of the printed object
9. For certain applications, it can be advantageous if the
reflecting layer of diffuse reflecting material 1 is immediately
adjacent to the bulb 16 of the UV radiation sources 8. This is
possible, in particular, with low pressure mercury gas discharge
lamps.
[0109] FIG. 7 shows a schematic cross section through a drier 20.
This drier 20 differs from the driers in accordance with FIGS. 5
and 6 in that the reflector 5 consists essentially of one or more
sheet metal reflectors which are not flat but curved in a concave
manner with respect to the UV radiation sources 8. The reflectors 5
can also be stationary and -an be disposed at a small separation
from the low pressure gas discharge lamps due to their low heat
production.
[0110] Perspective views are shown in FIGS. 8 through 10. FIG. 8
corresponds to FIG. 5, FIG. 9 to FIG. 6, and FIG. 10 to FIG. 7. In
all the figures, construction elements such as electrical leads,
cooling devices and mechanical supports are not shown for reasons
of clarity.
[0111] FIGS. 11 through 13 show modifications of FIGS. 8 through 10
which differ with regard to the transport direction 10 of the
printed material 9. In FIGS. 8 through 10, the printed material 9
is transported at right angles to the axial direction of the UV
radiation sources 8. With the driers 20 in accordance with FIGS. 11
through 13, transport occurs in the axial direction of the UV
radiation sources 8. In principle, the transport direction 10 can
assume any arbitrary angle with respect to the axes of the low
pressure gas discharge lamps. The transport directions 10 shown are
preferred for optimizing use of the emitted UV light and to achieve
an illumination time which is evenly distributed on the printed
material 9.
[0112] FIGS. 14 and 15 show schematic views of a plurality of UV
radiation sources 8 configured as U-shaped low pressure gas
discharge lamps 7. In the embodiment shown, a total of nine lamps
are disposed with mutually adjacent lengthwise sides for even
illumination of the drying surface of the printed material 9. In
addition, the lamps are alternately oppositely directed to effect
as compact an assembly as possible with high illumination
intensity. The electrical connection elements 13 therefore form an
alternating series with the closed ends of the U-shaped lamps at
both sides of the arrangement. There is a sufficient amount of
space between each of the electrical connection elements 13 such
that the separation between the lamps is not limited by the
electrical connection elements 13. FIGS. 14 and 15 differ with
regard to the transport direction 10 of the printed material 9
whose drying surface, having the UV printing ink 14 which is to be
cured, passes by the lamps. The reflectors are not shown in FIGS.
14 and 15.
[0113] FIG. 16 shows a schematic cross section through a drier 20
and a printing machine. Its UV radiation source 8 comprises a
conventional high pressure gas discharge lamp emitting in the UV
region. In addition to the lamps, the housing 27 also contains
pivoting reflectors 21 for directing the light onto the printed
material 9. When the installation comes to rest, these reflectors
21 can be pivoted to protect the printed material 9 from
overheating. A heat protection glass 22 is also provided for, since
the conventional UV illumination sources 8 generate a large amount
of heat. In accordance with the invention, one or more low pressure
gas discharge lamps should therefore be used as UV radiation source
8. The pivoting reflector 21 can then be a stationary reflector in
the
[0114] manner mentioned above and the heat protection glass 22 can
be eliminated. In this manner, the drier 20 can be of compact
construction and evenly illuminate the printed material 9. The heat
load is also substantially reduced.
[0115] In the example shown, the printed material 9 are tubes or
cups disposed on rotating tube arbors 26 of a tube plate 25. The UV
curing printing ink 14 is introduced from a wiping blade chamber or
a color chamber (not shown) to the printing apparatus with its
associated raster roller 23 and block roller 24. The block roller
24 transfers the pattern onto the tubes. The rotating tube plate 25
guides the tubes through the curing zone of the drier 20 to effect
curing via UV irradiation. After leaving the curing zone, the tubes
are removed from the tube arbors 26 and the tube arbors 26 are
provided with fresh non-printed tubes. The associated mounting and
removal devices are not shown.
[0116] In order to achieve a homogeneous illumination of the
printed material 9 within the curing zone as well as a high light
yield, the curing zone is surrounded with optically diffuse
reflecting material 1 in accordance with the invention. The
reflecting material 1 can be introduced onto special substrates 6
or disposed on the housing 27. The evenness of the illumination and
the light yield can be improved, in particular, by use of
reflectors 5 disposed on the side of the printed material 9 facing
away from the illumination source 8. If the heat produced by the
illumination source 8 is not excessively high, the pivoting
reflectors 21 can also be provided with material 1 in accordance
with the invention. A stationary reflector in accordance with the
invention can alternatively be disposed on the side of the
illumination source 8 facing away from the printed material 8.
[0117] FIG. 17 shows the drier of FIG. 16 in an embodiment in
accordance with the invention having low pressure gas discharge
lamps 7. The printed material 9 are tubes or cups disposed on
rotating tube arbors 26 of a tube plate 25. They are transported
through the drier 20 at a path speed of circa 50 m/min. In addition
to this motion along a path, the tube arbors 26 also rotate. The
drier 20 comprises a housing 27 in which the reflecting material 1
is disposed on substrates 6 for effecting a homogeneous
illumination of the printed material in the curing zone. The
reflector 5 provides for homogeneous illumination in combination
with the 12 low pressure gas discharge lamps 7. The low pressure
gas discharge lamps 7 are disposed at close proximity to another
and the printed material 9 is fed past and in close proximity to
the low pressure gas discharge lamps 7. Neither an expensive and
difficult cooling mechanism nor a heat protection glass are needed
due to the low heat production of the low pressure gas discharge
lamps 7. The reflector 5 is stationary and does not comprise any
pivoting components. FIG. 18 shows a detail of FIG. 17.
[0118] FIGS. 19, 20, and 21 show typical relative spectral
radiation fluxes of mercury gas lamps. FIGS. 19 and 20 each show
the spectral radiation flux E in arbitrary units as a function of
wavelength w and, in FIG. 21, in absolute units as a function of
wavelength w. FIG. 19 shows the spectrum of a high pressure lamp
and FIG. 20 that of a low pressure UV-C lamp. It can be seen that,
the UV-C low pressure gas discharge lamp emits primarily in the
UV-C region, whereas the main emission region of the high pressure
lamp is at longer wavelengths.
[0119] The UV-C low pressure gas discharge lamp of FIG. 20 is a low
pressure lamp which does not have any added fluorescent material,
e.g. it is a non-actinic low pressure lamp. FIG. 21 shows the
spectrum of a UB-B low pressure gas discharge lamp. This is a
fluorescent material lamp whose principal emissions are displaced
into the region near 305 nm through the addition of fluorescent
material. Further intensities also appear in the UV-A and visible
region.
* * * * *