Note: Descriptions are shown in the official language in which they were submitted.
.
2183007
POSTGLOS - 1 - X1302B
DESCRIPTi0h1
F;P7d_n~invPnt;on
This invention relates to an electrostatographic
printer and to a method of printing images.
Bankcfrcn,nd of thA ; nt i ~n
Electrostatographic printers are known in which
receiving material, such as paper, is fed along a path
past one or more image forming devices where toner
images are formed on at least one surface thereof. The
toner images are then fixed onto the receiving
material.
;5 Printers are known in which the receiving material is
in the form of a web. For example EP-A-629930 (Xeikon
NV) uses a receptor material in the form of a web while
EP-A-629924 (Xeikon NV) describes a multi-colour duplex
printer in which toner images are formed on both sides
of a web of receiving material. The toner images are
then fixed onto the receiving material by radiant
fixing devices.
The result of this printing method is to produce images
with a matt appearance and these images usually have a
low colour saturation. While this appearance is
2183007
,1
POSTGLOS - 2 - X1302B
acceptable for many applications, it is sometimes
desired to provide an image having a different
appearance or finish. By the term ~~finish° in the
context of the present invention, we mean either a
surface characteristic which is glossy, i.e. highly
reflective, or which provides high saturation of
colours, this usually being achieved by reducing the
scattering of light from the surface of the printed
article, or both such characteristics. For example, a
glossy appearance is especially desirable where the
receiving material itself has a glossy surface. A
higher degree of colour saturation can be very
desirable in high quality print work.
It has been proposed to provide glossy images by the
use of a toner which incorporates a glossing agent, or
by the application of a transparent glossing layer over
the toner image. However, these methods are costly in
terms of consumables.
Summa,-~r of the Tnvt~ntinn
It is an object of the present invention to provide a
printer, and a method of printing, in which toner -
images with modified finish can be produced in a simple
and convenient manner.
1 T
283007
POSTGLOS - 3 - X1302B
We have discovered that this objective can be achieved
by the treatment of the receiving material web in a
specified manner after passing the fixing device.
According to a first aspect of the invention, there is
provided an electrostatographic printer comprising:
means for feeding a web of receiving material along a
web path;
at least one toner image forming device for forming at
least one toner image on at least a first face of
receiving material as it is fed along the web path; and
fixing means for fixing the toner image onto the
receiving material,
nhararrPr;~Pd by at least one finishing element
positioned downstream of the fixing means, to contact
the first face of the receiving material while the
contacting surface of the toner image is at a
temperature above the glass transition temperature Tg
of the toner to modify the finish of the toner image.
We have found that it is not always possible to obtain
the desired toner finish by use of the fixing means
S T
283007
POSTGLOS - 4 - X1302B
alone, even if roller fixing is used. While not
wishing to be bound by theory, it appears that a
separate contact with a finishing element while the
toner surface is above the toner glass transition
temperature, especially together with moderate
pressure, is necessary to transform the toner surface
in a desirable manner.
Contact between the web and the finishing element is
l0 essential. We prefer a rolling contact, to avoid any
significant slip between the web and the finishing
element. Thus, the finishing element is preferably an
endless surface, such as the surface of a roller or a
continuous belt. In the following general description,
reference is made to finishing rollers, but it is to be
understood that such references are also applicable to
endless belts or to any other form of endless surface
means. We have found that an average contact pressure
over the contact area of from 2 to 20 N/cm', such as
from 5 to 10 N/cm2 is preferred. In particular, it is
preferred that the contacting force at each end of the
finishing element be from 20 to 200 N. The finishing
roller will usually be a pair of freely rotating, i.e.
non-driven rollers.
Where the finishing element is a finishing roller,
f f
2183007
POSTGLOS - 5 - X1302B
sufficient contact between the finishing roller and the
web may be achieved by providing a backing roller
biased into contact with the finishing roller to form a
nip there-between, through which the web passes. Two
or more such finishing rollers, each with its
associated backing roller, may be used. -Alternatively,
the geometry of the web path may be such that the web
contacts the finishing roller over a finite angle,
tension in the web being sufficient to generate the
necessary contact pressure.
The invention may be carried out according to one of
two possible principles. According to the first
principle, the finishing roller is positioned
immediately downstream of the fixing means and contacts
the web of receiving material while the temperature of
at least the top surface of the toner image is above
Tg. According to the second principle, the web of
receiving material is cooled after leaving the fixing
means to bring at least the top surface of the toner
image to a temperature below Tq, and the finishing
roller is heated to return at least the top surface of
the toner image to a temperature above Tg. The second
principle leads to an advantage in terms of the
temperature profile through the thickness of the
receiving material, finishing taking place while the
z r
283007
POSTGLOS - 6 - X1302B
temperature of the top surface of the toner image is
higher than that of the interior of the receiving
material.
Whether the finishing rollers are heated or cooled, the
printer may further comprise means for controlling the
heat output, or the cooling thereof, respectively. The
temperature of the rollers should not be so high that
toner is transferred from the receiving material to the
IO surface of the rollers, to be deposited on a following
location on the receiving material, thereby producing
hot offset artefacts. Thus, the temperature of the
rollers is preferably controlled not to exceed 10 C°
above the glass transition temperature (T9) of the
toner at the point where the paper breaks contact with
the roller surface.
In one embodiment of the first principle of the
invention, the finishing roller is positioned
immediately downstream of the fixing means. In this
embodiment the printer may further comprise cooling
means for cooling the finishing roller. Such cooling
means may comprise means for blowing cold air onto the
outside surface of the finishing roller.
Alternatively, in the case where the finishing roller
is hollow, a cooling fluid such as cold air or a liquid
2183007
POSTGLOS - 7 - X13~2B
coolant may be passed through the hollow interior of
the roller. As a further alternative, a cooling liquid
such as purified water may be applied to the outside
surface of the finishing roller.
The invention is applicable to printers, such as multi-
colour printers, comprising a plurality of toner image
forming devices positioned on the same side of the,web
path for forming overlapping images on the first face
of the receiving material, the fixing means being
positioned downstream of the toner image forming
devices.
The invention is also particularly applicable to
printers capable of forming toner images on both faces
of a web of receiving material, i.e. "duplex" printers.
Such printers preferably comprise two toner image
forming devices positioned one on either side of the
web path for forming images on both faces of the
receiving material, and according to the invention
further comprise a pair of finishing rollers with the
web path extending between them.
The pair of finishing rollers may be provided adjacent
one another and biased towards each other to define a
nip there-between, through which the web path extends
2~g3007
POSTGLOS - 8 - X1302B
and to provide the necessary contact pressure. This
arrangement is particularly convenient where the
finishing rollers are located immediately following the
radiant fixing means, and the web of receiving material
is to contact the finishing rollers before the
temperature of the toner falls below its glass
transition temperature (Tg).
Alternatively, one of the pair of finishing rollers may
be provided upstream of the other, the web taking a
serpentine path around these rollers. In this case the
necessary contact pressure derives from the tension in
the web and the geometry of the arrangement. This
arrangement is particularly convenient when the web
contacts the first finishing roller before the
temperature of the toner falls below its glass
transition temperature (T9) and an intermediate web
heating device is provided between the finishing
rollers. However, this arrangement is generally more
demanding on space.
Whether the finishing rollers are provided adjacent
each other, or spaced one upstream from the other, the
web needs to contact the surface thereof over a finite
distance, to allow sufficient time for the top surface
of the toner to cool to a temperature low enough to
2183007
POSTGLOS - 9 - X1302B
allow easy release from the finishing rollers. The
optimum length of this contact distance will depend
inter alia upon the speed of the moving web and its
temperature profile as it makes and breaks contact with
the finishing element. Where the finishing element is
a belt, the required contact distance is provided by
the length of that part of the belt which is contacted
by the web. Where the finishing element is a roller,
the required contact distance can be achieved by the
web being in contact therewith over a finite contact
angle. Alternatively, in the case of a pair of rollers
biased together to form a nip, the required contact
distance can be achieved by ensuring that the
elastomeric properties of the rollers and the bias
force are such as to form a nip of sufficient length.
Where the web starts making contact with the finishing
rollers while the web is at a temperature above the
glass transition temperature T9, this temperature is
preferably at least 45 C° above Tg, typically within
the range of 100 to 140 °C. The web should however
break contact with the finishing rollers at a
temperature which is preferably not more than 10 C°
above Tg, typically within the range of 50 °C to 95 °C.
In an embodiment according to the second principle, the
finishing roller is a heated finishing roller.
218307
POSTGLOS - 10 - X1302B
According to an embodiment of the second principle of
the invention, the printer further comprises cooling
means positioned between the fixing means and the
finishing elements to cool the top surface of the toner
image, thereby to reduce or prevent hot offset, for
example to cool the top surface of the toner image to a
temperature below the glass transition temperature T9
of the toner prior to contact with the finishing
element and further comprises heating means for heating
the finishing means.
In this alternative embodiment, particularly adapted
for use with nduplex" printing, the web leaving the
fixing station may pass over a pair of cooling rollers
whereby each face of the web is cooled so as to bring
the temperature of the toner image thereon below the
glass transition temperature of the toner. These
cooling rollers may be hollow earthed metal rollers
carrying a conductive elastomeric coating such as
silicone rubber. Fans may be provided to blow cold air
through the interiors of such hollow cooling rollers.
In this embodiment, the finishing station may comprise
two parts, a first pair of finishing rollers being
provided at the entry of the finishing station and a
second pair of finishing rollers being provided at the
2183007
POSTGLOS - 11 - X1302B
exit of the finishing station. The housing parts may
be biased towards each other, for example by springs,
thereby biasing the finishing rollers of each pair into
contact with each other to form a nip through which the
web extends.
While the finishing roller may be heated by exposing
the outside surface thereof to heating means, in the
case of a hollow finishing roller an alternative is to
pass a heating fluid through the hollow interior of the
roller, or to mount a radiant heating device within the
hollow interior. For example, each finishing roller may
be provided with heating means such as a radiant
heater. Temperature sensors may be provided to measure
the temperature of the surface of at least one
finishing roller of each pair. These temperature
sensors are preferably coupled to the controllable
power supply means for the radiant heaters.
2~ A particular advantage may be gained by external
heating of the finishing rollers, especially where they
are made of a generally heat non-conductive material,
such as silicone rubber. By external heating, the
temperature of the interior of the roller is lower than
would be the case with internal heating. The silicone
rubber is thereby subjected to lower thermal stress,
2183007
POSTGLOS - 12 - X13028
resulting in longer lifetime and/or avoiding the need
to include any heat conductive ingredients in the
silicone material. Response times are also faster,
enabling more accurate control of the roller surface
temperature.
While the present invention reduces the use of
consumables, the application of a release agent to the
finishing rollers is not excluded. Thus; each
finishing roller may optionally also be provided with a
lubricating roller for applying a release agent, such
as a silicone oil, to the surface of the finishing
rollers. The temperature of the surface of the
finishing rollers is so controlled as to contact the
surface of the web at a temperature above the glass
transition temperature Tg of the toner to modify the
finish of the toner image.
The printer may also further comprise means for cooling
the opposite face of the receiving material while the
first face of the receiving material contacts the
finishing element. Thus, in a modification of the
embodiment referred to above, the web leaving the
fixing station firstly passes over an advance cooling
roller, whereby one face of the web is cooled to bring
the temperature of the toner image thereon below the
2183007
POSTGLOS - 13 - X1302B
glass transition temperature of the toner. The web
then enters the finishing station which comprises two
housing parts, in each of which can be found a pair of
finishing rollers and an opposed cooling roller. The
housing parts may be biased towards a fixed part of the
apparatus, for example by springs, thereby biasing the
finishing rollers into contact with the opposed cooling
roller to form nips through which the web extends.
Where one finishing roller is upstream of the other,
i.e. the finishing rollers are not biased towards each
other to form a nip there-between, each finishing
roller may be provided with electrostatic charging
means, such as a corona, to provide an electrostatic
charge on the surface of the receptor material as it
makes contact with the finishing roller. This
arrangement increases the force with which the receptor
material is held against the surface of the finishing
roller. Cleaning means, such as a cleaning brush, may
also be provided to remove any toner particles from the
surface of the finishing roller after release of the
receptor material from the surface thereof.
Usually web cooling means will be positioned downstream
of the finishing rollers, ideally immediately
downstream thereof, so that the temperature of the
'i 2183007
POSTGLOS - 14 - X1302B
toner is reduced below the glass transition temperature
(Tg) of the toner as quickly as possible. However,
where the finishing rollers are cooled, separate web
cooling means may prove to be unnecessazy.
Preferably, the finishing roller is provided with a
non-adhesive, i.e. abhesive web-contacting surface,
such as a smooth metal surface or a surface coated with
polytetrafluoroethylene (e.g. TEFLON - Trade Mark - ex
Dupont).
The finishing rollers may comprise a rigid core
provided with a covering of elastomeric material, such
as silicone rubber or EPDM (ethylene-propylene
elastomer).
In a preferred embodiment of the invention, the toner
image forming device comprises at least one imaging
station positioned adjacent the web path and including
rotatable endless surface means, means for applying an
electrostatic latent image to the rotatable endless
surface means, developing means for developing the
electrostatic latent image into a toner image (usually
by way of magnetic brush technology), and transfer
means for transferring the toner image from the endless
rotatable surface means to the receiving material.
283007
POSTGLOS - 15 - X1302B
In a preferred embodiment of the present invention, the
printer comprises a plurality of toner image forming
devices, adapted to apply images of different colours
onto the receiving material, i.e. a multi-colour
printer.
The developing means comprises a developer which
contains toner particles containing a mixture of a
resin, a dye or pigment of the appropriate colour and
normally a charge-controlling compound giving
triboelectric charge to the toner. In dual-component
developers which are normally used, carrier particles
are also present for charging the toner particles by
frictional contact therewith. The carrier particles
may be made of a magnetizable material, such as iron or
iron oxide. In mono-component developers, the carrier
particles are not present, the toner particles
themselves containing a magnetisable material, or else
a developing technology other than magnetic brush
development is used.
Dry-development toners essentially comprise a
thermoplastic binder consisting of a thermoplastic
resin or mixture of resins including colouring matter,
e.g. carbon black or colouring material such as finely
dispersed dye pigments or soluble dyes.
2183007
POSTGLOS - 16 - X1302B
The mean diameter of dry toner particles for use in
magnetic brush development is about 10 um (ref.
"Principles of Non Impact Printing" by Jerome L.
Johnson - Palatino Press Irvine CA, 92715 U.S.A.
(1986), p. 64-85), but may be from 1 to 5 um for high
resolution development (see e.g. British patent
specification GB-A-2180948 and International patent
specification WO-A-91/00548).
The toner particles contain in the resinous binder a
colorant (dissolved dye or dispersed pigment) which may
be white or black or has a colour of the visible
spectrum, not excluding however the presence of infra-
red or ultra-violet absorbing substances and substances
that produce black in admixture.
White toner images may be used on coloured printing
stock or transparent receptor material e.g. for the
production of back-lightened transparent advertising
panels.
The thermoplastic resinous binder may be formed of
polyester, polyethylene, polystyrene and copolymers
thereof, e.g. styrene-acrylic resin, styrene-butadiene
resin, acrylate and methacrylate resins, polyvinyl
chloride resin, vinyl acetate resin, copoly(vinyl
X1$3007
POSTGLOS - 17 - X1302B
chloride-vinyl acetate) resin, copoly(vinyl chloride-
vinyl acetate-malefic acid) resin, vinyl butyral resins,
polyvinyl alcohol resins, polyurethane resins,
polyimide resins, polyamide resins and polyester
resins. Polyester resins are preferred for providing
high gloss and improved abrasion resistance. Such
resins usually have a glass transition point of more
than 45°C with a melt viscosity of at least 500 poise
up to no more than 15000 poise. The presence of other
ingredients in the toner particles, such as the
colorant, usually have no significant effect upon the
glass transition temperature. The volume resistivity
of the resins is preferably at least 101' n-cm.
Suitable toner compositions are described in European
patent applications EP-A-601235, and EP-A-628883 and
International patent applications WO 94/27192, 94/27191
and 94/29770 tall Agfa-Gevaert NV). The glass
transition temperatures of most common toner
compositions are similar at about 55°C. Where the web
carries a number of different toners, as for example in
the case of multi-colour images, the finishing element
should make contact with the web while the web is at a
temperature above the lowest glass transition
temperature of the toners present, most preferably
above the highest glass transition temperature of the
r
283007
POSTGLOS - 18 - X1302B
toners present, and preferably break contact near the
lowest glass transition temperature.
According to a second aspect of the invention, there is
provided a method of forming a toner image on a web of
receiving material, comprising:
feeding a web of receiving material along a web path;
forming at least one toner image on at least a first
face of the receiving material as it is fed along the
web path; and
fixing the toner image onto the receiving material,
nharanrPr;sPd h~~ thereafter contacting the first face
of the receiving material with at least one finishing
element while the top surface of the toner image is at
a temperature above the glass transition temperature
(TQ) of the toner, to modify the finish of the toner
image.
In an embodiment of the printer according to the
invention, means are provided to enable images to be
produced selectively without a glossy appearance, where
this may be desired. For example, in the case of
Y
2~g3007
POSTGLOS - 19 - X1302B
finishing rollers which contact the receiving material
before the temperature thereof falls below the glass
transition temperature of the toner, means may be
provided for selectively moving these rollers out of
contact with the web. In the case of heated finishing
rollers, means may be provided for selectively
switching off the supply of heat energy thereto.
A number of fixing methods are known. The fixing means
may take a variety of forma such as hot roller or
radiant fixing.
For example, the web carrying the toner image is passed
over heated rollers. A problem arises with such roller
fixing, that if the temperature of the rollers is too
high, toner is transferred from the receiving material
to the surface of the rollers (an effect known as "hot-
offset"), to be deposited on a following location on
the receiving material, thereby producing the effect of
°°ghost" images. Ghosting can be reduced by
continuously applying a release agent, such as a
silicone oil, to the surface of the rollers, but this
release agent represents a consumable material which
adds to the running costs of the printer. If the
temperature of the rollers is too low, the toner is
insufficiently fixed to the paper and an effect known
2183007
POSTGLOS - 20 - X1302B
as ~~cold offset" can occur. After a period of time the
heated rollers may become subject to wear. Also, due
to the fact that heating occurs with contact with the
receptor material, disadvantageous surface effects can
occur. Furthermore, when the paper is in sheet form,
the paper tends to adhere to the rollers and special
paper stripping means need to be provided and the
choice of materials for forming the surface of the
rollers is limited.
Nevertheless, we have found that, where the web carries
a toner image on one face only, i.e. so-called
"simplex" printers, the fixing means may suitably be a
heated fixing roller formed of a rigid material, e.g, a
metal roller, having a surface coating of an abhesive
material such as a silicone rubber, without the need to
apply a release agent. An opposed pressure roller
formed for example of rubber or an elastomeric
material, is provided, biased towards the fixing roller
to form a nip there-between through which the web
passes. This fixing arrangement is particularly
beneficial when the web comprises a low-melting point
material, such as a plastics material commonly used as
a substrate for printed labels, which is liable to
stretch if passed through a radiant fixing device. The
temperature of the heated fixing roller, which may be
2183007
POSTGLOS - 21 - X1302B
monitored by the use of an appropriately positioned
sensor, may be so controlled as to incompletely fix the
toner image, fixing being completed by contact with a
heated finishing element.
A number of radiant fixing techniques are also known.
In "flash-fixing" a short intense burst of radiant
energy is applied to the receptor material carrying the
toner image to be fixed. The wavelength of the
radiant energy is chosen to be absorbed by the toner
and is therefore in the visible or ultra-violet part of
the spectrum. Such a technique is unsuitable for
multi-colour images, where toners of different
composition are carried on the receptor material, said
toners having different absorption characteristics in
the visible spectrum.
A number of constructions of image fixing stations have
been proposed in the art which use a radiant heat i.e.
infra-red fixing technique. Thus, for example,
European patent application EP-A-629930 (Xeikon NV)
describes an image-fixing station for fixing the toner
image on a moving receiving material, which comprises
two pairs of radiant heat sources, the peak energy
output wavelength of which lies in the non-visible part
of the spectrum.
~~~30~1
POSTGLOS - 22 - X1302B
By the use of infra-red radiant fixing the whole
receptor material i.e. not only the toner, becomes hot.
Most receptor materials have a relatively high heat
capacity and therefore take some time to cool. For
this reason a cooling device, especially a non-contact
cooling device is often provided downstream of the
fixing station.
When the web of receiving material is formed of paper,
a dangerous situation can arise in the event of a
failure of the web drive, such as may occur if there is
an electrical power failure. Even if electrical power
to the infra-red radiant fixing device is cut at this
time, a portion of the web stands in close proximity to
the radiant elements which may retain significant heat
for some time. There is therefore a risk that the web
material will be damaged or may even catch fire, and
further that such a fire may spread along the web to
other parts of the printer.
An embodiment is described in European patent
application EP-A-629930 (Xeikon NV) in which the
radiant sources are located in hingedly mounted pairs
of housings in such a manner that the housings may be
moved into a closed position to shield the radiant
sources from the web when the speed of movement of the
2 ~ 83007
POSTGLOS - 23 - X1302B
web falls below a predetermined value. Damage to the
web when movement thereof stops is thereby avoided.
We prefer to use a non-contact fixing method and in
particular infra-red radiant fixing. The peak energy
output wavelength of the radiant source preferably
corresponds to an absorption wavelength of the receptor
material, which in preferred embodiments of the
invention comprises paper, which absorbs radiation
strongly (i.e. absorbs more than 50% incident energy)
over the range of 3~Cm to at least S~Cm. Thereby, the
toner particles are heated indirectly, from the heat
energy absorbed by the receptor material, the heat
energy being transferred from the receptor material by
a combination of conduction and convection. Some heat
energy will, of course, be absorbed directly by the
toner particles, to a degree depending upon their
composition. Thus, the radiant source may comprise
one or more infrared emitting sources, causing the
receptor material to be heated above the melting
temperature of the toner particles, so that the latter
melt and adhere thereto. Due to the fact that heating
occurs without contact with the receptor material,
calendering effects are avoided. The radiant source
preferably has a radiant energy output wavelength
within the range of from 1 to 10 Vim, such as within the
2183~J07
POSTGLOS - 24 - X1302B
range of from 3 to 6 ~Cm.
The radiant heat sources may be provided along a
relatively long trajectory of the receptor material, so
that less restriction is placed upon the speed of the
latter. Furthermore, smearing of the transferred
toner image is also avoided. The infrared emitting
radiant sources are preferably such as emit heat by the
Joule-effect.
It is an additional objective of the invention, where
infra-red radiant fixing is used, to reduce the risks
of the spread of fire in the printer, in the event of a
failure of web transport, whether the radiant fixing
device is of the type describes in EP-A-629930 or some
other type.
Thus, according to a preferred feature of the
invention, the fixing means comprises infra-red radiant
fixing means comprising a fixer housing having a Web
inlet, a web outlet and infra-red radiant elements, and
a finishing roller pair are located immediately
downstream of the web outlet, through which the web
path extends, and means are provided for moving the
rollers of the finishing roller pair between an open
position in which the rollers are not in contact with
2183007
1
POSTGLOS - 25 - X1302B
the receiving material and a closed position in which
the rollers are in contact with the receiving material
and are positioned to close the web outlet.
In this embodiment of the invention, the printer may
further comprise means for automatically closing the
finishing roller pair when movement of the web falls
below a predetermined threshold, such as below 25~ of
nominal speed. This automatic closure of the fixer
housing may also be triggered automatically in the
event of a power failure.
One suitable embodiment is where the rollers are
mounted on articulated arms, biasing means acting on
the articulated arms being provided to bias the rollers
towards each other and drive means acting on the
articulated arms being provided to move the rollers
away from each other. A releasable coupling may be
provided between the drive means and the articulated
arms, to decouple the drive means therefrom when
movement of the web falls below a predetermined
threshold.
A further roller pair may be positioned immediately
upstream of the web inlet, through which the web path
passes. This further roller pair may also be arranged
2183001
POSTGLOS - 26 - X1302B
to close when the web speed falls.
The arrangement whereby the finishing rollers can be
moved between open and closed positions can be
particularly beneficial when the finishing rollers are
driven rollers, heated from outside. At start-up of
the printer, the rollers may be retained in the open
position while being heated up to operating
temperature, and thereafter moved into the closed
operating position.
Pre d PmhodimPnYa of th invPn iQn
The invention will now be further described, purely by
way of example, with reference to the accompanying
drawings in which:
Figure 1 shows a section of a printer according to an
embodiment of the invention, capable of simultaneous
duplex printing;
2a
Figure 2 shows in detail a cross-section of one of the
print stations of the printer shown in Figure 1;
Figure 3 shows details of the finishing station of the
printer shown in Figures 1 and 2;
2183007
POSTGLOS - 27 - X1302B
Figure 4 shows details of a finishing station of a
printer according to an alternative embodiment of the
invention;
Figure 5 shows details of the finishing roller of the
finishing station shown in Figure 4;
Figure 6 shows details of a finishing station according
to a further alternative embodiment of the invention;
l0
Figures 7a, 7b and 7c show details of the mounting and
operating mechanism of the finishing station shown in
Figure 3, in different stages of operation;
Figure 8 shows an alternative embodiment of a finishing
station according to the invention; and
Figure 9 shows a modification of the embodiment shown
in Figure 8.
The printer 10 shown in the Figures has a supply
station 13 in which a roll 14 of web material 12 is
housed, in sufficient quantity to print, say, up to
5,000 images. The web 12 is conveyed into a tower-
like printer housing 44 in which support columns 46 and
46~ are provided, each housing five similar printing
2183007
POSTGLOS - 28 - X13028
stations A to E and A' to E'. The image-producing
stations A, B, C and D and likewise A', B', C' and D'
are arranged to print yellow, magenta, cyan and black
images respectively. The stations E and E' are
provided in order to optionally print an additional
colour, for example a specially customised colour, for
example white. Each sub-group of printing stations A
to E and A' to E' are mounted in a substantially
vertical configuration resulting in a reduced
footprint. The columns 46 and 46' may be mounted
against vibrations by means of a platform 48 resting on
springs 50, 51. The columns 46 and 46' may be mounted
on rails enabling their relative movement. In this way
the columns may be moved away from each other for
servicing purposes.
The web of paper 12 unwound from a supply roller 14 is
conveyed in an upwards direction past the printing
stations in turn. A brake 11 acts against the roller
14. After passing the last printing station E', the
web of paper l2 passes over a reversing roller 150 and
then downwards through an image-fixing station 16, a
finishing station 17 (which for the sake of clarity is
not shown in Figure 1), a cooling station 18 and thence
to a cutting station 20 to cut the web 12 into sheets
and to a cut sheet stack 52. The web 12 is conveyed
2183007
POSTGLOS - 29 - X1302B
through the printer by two drive rollers 22a, 22b one
positioned between the supply station 13 and the first
image-producing station A and the second positioned
between the cooling station 18 and the cutting station
S 20. The drive rollers 22a, 22b are driven by
controllable motors, 23a, 23b. One of the motors 23a,
23b is speed controlled at such a rotational speed as
to convey the web through the printer at the required
speed, which may for example be about 125mm/sec. The
other motor is torque controlled in such a way as to
generate a web tension of, for example, about 1 N/cm
web width.
As shown in Figure 2, each image-producing station
I5 (e.g. printing station B) comprises a cylindrical drum
24 having a photoconductive outer surface 26.
Circumferentially arranged around the drum 24 there is
a main corotron or scorotron charging device 28 capable
of uniformly charging the drum surface 26, for example
to a potential of about -600V, an exposure station 30
which may, for example, be in the form of a scanning
laser beam or an LED array, which will image-wise and
line-wise expose the photoconductive drum surface 26
causing the charge on the latter to be selectively
dissipated, for example to a potential of about -250V,
leaving an image-wise distribution of electric charge
', 2183007
POSTGLOS - 30 - X1302B
to remain on the drum surface 26. This so-called
"latent image" is rendered visible by a developing
station 32 which by means known in the art will bring a
developer in contact with the drum surface 26. The
developing station 32 includes a developer drum 33
which is adjustably mounted, enabling it to be moved
radially towards or away from the drum 24 for reasons
as will be explained further below. In a typical
construction of a developer station, the developer drum
33 contains magnets carried within a rotating sleeve
causing the mixture of toner and magnetizable carrier
particles to rotate therewith, to contact the surface
26 of the drum 24 in a brush-like manner. Negatively
charged toner particles, triboelectrically charged to a
level of, for example 9 uC/g, are attracted to the
photo-exposed areas on the drum surface 26 by the
electric field between these areas and the negatively
electrically biased developer so that the latent image
becomes visible.
After development, the toner image adhering to the drum
surface 26 is transferred to the moving web 12 by a
transfer corona device 34. The moving web 12 is in
face-to-face contact with the drum surface 26 over a
wrapping angle ~ of about 15° determined by the
position of the drums 24' and 24 " of the immediately
2183001
POSTGLOS - 31 - X1302B
adjacent opposed printing stations (e. g. printing
stations A' and B'). As will be seen from Figure 1,
the columns 46 and 46' are mounted closely together so
that the web 12 travels in a generally vertical path
defined by the facing surfaces of the imaging station
drums 24, 24'. This arrangement is such that each
imaging station drum acts as the guide roller for each
adjacent drum by defining the wrapping angle.
The transfer corona device, being on the opposite side
of the web to the drum, and having a high potential
opposite in sign to that of the charge on the toner
particles, attracts the toner particles away from the
drum surface 26 and onto the surface of the web 12.
The transfer corona device typically has its corona
wire positioned about 7 mm from the housing which
surrounds it and 7 mm from the paper web. A typical
transfer corona current is about 3~tA/cm web width.
The transfer corona device 34 also serves to generate a
strong adherent force between the web 12 and the drum
surface 26, causing the latter to be rotated in
synchronism with the movement of the web 12 and urging
the toner particles into firm contact with the surface
of the web 12. The web, however, should not wrap
around the drum beyond the point dictated by the
positioning of the drum of the next adjacent printing
2183007
POSTGLOS - 32 - X1302B
station and there is therefore provided
circumferentially beyond the transfer corona device 34
a web discharge corona device 38 driven by alternating
current and serving to discharge the web 12 and thereby
allow the web to become released from the drum surface
26. The web discharge corona device 38 also serves to
eliminate sparking as the web leaves the surface 26 of
the drum.
Thereafter, the drum surface 26 is pre-charged to a
level of, for example -580V, by a pre-charging corotron
or scorotron device 40. The pre-charging makes the
final charging by the corona 28 easier. Any residual
toner which might still'cling to the drum surface may
be more easily removed by a cleaning unit 42 known in
the art. Final traces of the preceding electrostatic
image are erased by the corona 28. The cleaning unit
42 includes an adjustably mounted cleaning brush 43,
the position of which can be adjusted towards or away
from the drum surface.26 to ensure optimum cleaning.
The cleaning brush is earthed or subject to such a
potential with respect to the drum as to attract the
residual toner particles away from the drum surface.
After cleaning, the drum surface is ready for another
recording cycle.
283001
POSTGLOS - 33 - X1302B
Referring to both Figures 1 and 2, after passing the
first printing station A (of a printer 10 - see Figure
1?, the web passes successively to image-producing
stations B, C and D, where images in other colours are
transferred to the web. It is critical that the
images produced in successive stations be in register
with each other. In order to achieve this, the start
of the imaging process at each station has to be
critically timed. However, accurate registering of
the images is possible only if there is no slip between
the web 12 and the drum surface 26.
The electrostatic adherent force between the web and
the drum generated by the transfer corona device 34,
the wrapping angle ~ determined by the relative
position of the drum 24 and the drums 24' and 24 " , and
the tension in the web generated by the drive roller 22
and the braking effect of the brake 11 are such as to
ensure that the rotational speed of the drum 24 is
determined substantially only by the movement of the
web 12, thereby ensuring that the drum surface moves
synchronously with the web.
The cleaning unit 42 includes a rotatable cleaning
brush 43 which is driven to rotate in a sense the same
as that of the drum 24 and at a peripheral speed of,
2P83pOl
POSTGLOS - 34 - X1302B
for example twice the peripheral speed of the drum
surface. The developing unit 32 includes a brush-like
developer drum 33 which rotates in the opposite sense
to that of the drum 24. The resultant torque applied
to the drum 24 by the rotating developing brush 33 and
the counter-rotating cleaning brush 43 is adjusted to
be close to zero, thereby ensuring that the only torque
applied to the drum is derived from the adherent force
between the drum 24 and the web 12. Adjustment of
this resultant force is possible by virtue of the
adjustable mounting of the cleaning brush 43 and/or the
developing brush 33 and the brush characteristics.
Figure 3 shows details of one embodiment of a finishing
station 17, which is positioned between the image
fixing station 16 and the cooling station 18.
Aa can be seen in Figure 3, the image fixing station
comprises a housing 60, which constitutes a fire wall,
the housing having an upper web inlet 62, a lower web
outlet 64 and control2able radiant elements 63.
Positioned immediately downstream of the web outlet 64
is a pair of finishing rollers 66, 67, which are biased
together by means not shown in Figure 3, to form a nip
through which the web 12 passes. The finishing rollers
66, 67 are cooled by controllable cold air fans 68, 69
2183007
POSTGLOS - 35 - X1302B
respectively.
As it leaves the fixing station 16 the web 12 is at a
temperature above the glass transition temperature Tg
of the toner which makes up the image thereon. Toner
on the faces of the web contacts the finishing rollers
66, 67 where the surface characteristics thereof become
modified to increase the gloss and improve the colour
saturation.
In the alternative embodiment shown in Figure 4, the
web 12 leaving the outlet 64 of the fixer housing 60,
follows a serpentine path between two spaced apart
finishing rollers 70, 71 en route to the cooling
station 18. As further shown in Figure 5, the
finishing roller 70 comprises an earthed metallic (e. g.
stainless steel) core 78 provided with a, preferably
conductive, elastomeric covering 80, for example formed
of silicone rubber, which may in turn carry a coating
of polytetrafluoroethylene, or other low friction
material. A DC corona device 72 operating at a voltage
of about 2 kV with a current of about 10 to 100 Ecp.
establishes a charge on the adjacent face of the web
12, which serves to attract the web towards the
finishing roller 70, thereby increasing the pressure
applied by that finishing roller to the web. An AC
~
2183007
POSTGLOS - 36 - X1302B
corona 73, operating at say 15 kV, removes the charge
from the web 12 as it breaks contact with the finishing
roller 70. The finishing roller 71 is similarly
constructed and arranged. As shown in Figure 4, each
of the finishing rollers 70, 71 may be provided with a
rotating brush 74 which serves to remove any stray
toner particles which may become adhered to the roller
surface, or may alternatively be used to apply a
release agent to the roller surface.
As it leaves the fixing station 16 the web 12 is at a
temperature above the glass transition temperature Tg
of the toner which makes up the image thereon. Toner
on the right hand face of the web as viewed in Figure 4
contacts the finishing roller 70 where the surface
characteristics thereof become modified to increase the
gloss and improve the colour saturation. Toner on the
left hand face of the web remains substantially
unaffected. Since the web leaves the finishing roller
70 at a temperature which is below Tg, an intermediate
heating device 76 is provided to increase the web
temperature above TQ before the toner which makes up
the image on the left hand side of the web contacts the
finishing roller 71, where its surface characteristics
are similarly modified.
~
2183007
POSTGLOS - 37 - X1302B
In the embodiment shown in Figure 6, positioned
immediately downstream of the web outlet 64 is a pair
of finishing belts 84, 85 which pass over upper
controllably heated rollers 82, 83 and lower cooled
rollers 86, 87 respectively. The geometric arrangement
of these belt-supporting rollers is such as to
establish pressurised contact between the belts 84 and
85. Alternatively, a pair of intermediate pressure
rollers biased together to form a nip through which the
finishing belts 84, 85 pass, may be provided. The
belts are formed of metal such as stainless steel,
provided With a PTFE coating, or of a heat conductive
polymer such as KAPTON (Trade Mark ex Dupont Corp.).
As it leaves the fixing station 16 the web 12 is at a
temperature above the glass transition temperature Tg
of the toner which makes up the image thereon. Toner
on the faces of the web contacts the finishing belts
84, 85 where the surface characteristics thereof become
modified to increase the gloss and improve the colour
saturation. The use of finishing belts in the
embodiment shown in Figure 6, rather than finishing
rollers, provides a longer contact time and thereby a
higher gloss is imparted to the toner image.
Referring to Figures 7a, 7b and 7c, it will be seen
~~~3007
POSTGLOS - ~8 - X1302B
that the finishing rollers 66, 67 are mounted on
articulated arms 88, 89, pivoted together at a pivot
point 90 in a scissors-like arrangement. A spring 92
urges the lower ends of the arms 88, 89 away from each
other to bias the rollers 66, 67 into contact. A
Bowden cable 94 is operatively connected to the arms
88, 89. While Figures 7a, 7b and 7c, show the
arrangement at only one end of the rollers 66, 67 it is
to be understood that the arrangement at the other end
is similar. The Bowden cables from both ends are
operatively connected to an actuating device 102. The
actuating device 102 comprises a linear drive motor 96,
which is fixed at a point 97 and has a moving shaft 99.
The shaft 99 carries a solenoid operated coupling
device 98, which is such that when current flows
through the solenoid thereof a slidable U-shaped frame
100 is attracted thereto. The frame 100 is connected
to both Bowden cables 94.
A control device (not shown) is provided for sensing
movement of the web 12 and cutting power to the
coupling device 98 if the web speed should fall below a
pre-set threshold, such as 25% of nominal speed.
Figures 7a and 7b show two normal operating positions,
in both of which current is flowing through the
2183007
POSTGLOS - ~9 - X1302B
solenoid of the coupling device 98. In Figure 7a, the
drive device 96 has been operated to retract the shaft
99, pulling the frame 100 to the right as seen in the
Figures, causing the Bowden cable to pull the arms 88,
89 together, against the force of the spring 92,
thereby opening the rollers 66, 67. This position
corresponds to an image non-finishing mode of the
printer.
In Figure 7b, the drive device 96 has been operated to
extend the shaft 99, urging the frame 100 to the left
causing the Bowden cable to release the arms 88, 89,
thereby allowing the rollers 66, 67 to close under the
force of the spring 92. This position corresponds to
the image finishing mode of the printer.
Figure 7c shows the position when movement of the web
12 has fallen below the predetermined threshold, or
there has been a power failure. In this position,
current has ceased to flow through the solenoid of the
coupling device 98 and the frame 100 has therefore been
released therefrom. This allows the frame 100 to slide
to the left causing the Bowden cables to release the
arms 88, 89, thereby allowing the rollers 66, 67 to
close under the force of the springs 92. The outlet 64
of the fixer housing is thereby closed. This position
2183007
POSTGLOS - 40 - X1302B
corresponds to the safety shut-down mode of the
printer, any fire in the fixer housing being safely
retained therein.
In the alternative embodiment shown in Figure 8, the
web 12 leaving the fixing station 16, at a temperature
of, for example 110 to 150°C, passes over a pair of
cooling rollers 201, 202, whereby each face of the web
is cooled sufficiently to prevent hot offset, for
example so as to bring the temperature of the toner
image thereon below the glass transition temperature of
the toner, for example below 65°C. These cooling
rollers are hollow earthed stainless steel rollers
carrying a conductive elastomeric coating of silicone
rubber, fans 203, 204 being provided to blow cold air
through the interiors thereof.
The web 12 now enters a finishing station 217. The
finishing station 217 comprises two housing parts 218,
219. A first pair of finishing rollers 220, 221 are
provided at the entry of the finishing station and a
second pair of finishing rollers 222, 223 are provided
at the exit of the finishing station. The finishing
rollers 220, 222 axe carried in one housing part 218,
while the finishing rollers 221, 223 are carried in the
other housing part 219. The housing parts 218, 219 are
2183007
POSTGLOS - 41 - X1302B
biased towards each other by springs 224, 225, thereby
biasing the finishing rollers of each pair into contact
with each other to form a nip through Which the web 12
extends.
Each finishing roller 220, 221, 222 and 223 is provided
respectively with a radiant heater 230, 231, 232, 233
to which electrical power is controllably supplied by
means not shown, the radiant heaters each having a heat
output under steady state conditions of from 50 to 200
Watts. Temperature sensors 234, 235 are provided to
measure the temperature of the surface of at least one
finishing roller of each pair. These temperature
sensors are coupled to the controllable power supply
means for the radiant heaters 230, 231, 232, 233. Each
finishing roller 220, 221, 222 and 223 is also provided
respectively with an optional lubricating roller 240,
241, 242, 243 for applying a release agent, such as a
silicone oil, to the surface of the finishing rollers.
The temperature of the surface of the finishing rollers
220, 221, 222 and 223 is so controlled as to contact
the surface of the web 12 at a temperature above the
glass transition temperature Tg of the toner to modify
the finish of the toner image. For example, the
temperature of the first pair of finishing rollers 220,
221 is from 80 to 110°C, while the temperature of the
2183007
POSTGLOS - 42 - X1302B
second pair of finishing rollers 222, 223 is from 90 to
120°C. In order to prevent hot offset at the second
pair of finishing rollers, 222, 223, it is important
that the temperature of the surface of these finishing
rollers is above that of the top surface of the toner
image. Although some cooling of the toner image can be
expected between the first and second pairs of
finishing rollers, we prefer to ensure that hot offset
is prevented by setting the temperature of the second
pair of finishing rollers slightly above that of the
first pair.
After leaving the finishing station 217, the web 12
passes over a further hollow cooling roller 250, to
cool the toner image to, for example, below 65°C, the
cooling roller 250 being provided with a fan 251 to
blow cold air through the interior thereof.
The embodiment shown in Figure 9 is somewhat similar to
that shown in Figure 8, but with a different spacial
arrangement.
In the alternative embodiment shown in Figure 9, the
web leaving the fixing station 16 firstly passes over
an advance cooling roller 300, whereby one face of the
web is cooled sufficiently to prevent hot offset, for
2183007
POSTGLOS - 43 - X1302B
example so as to bring the temperature of the toner
image thereon below the glass transition temperature of
the toner, for example below 65°C. The web then enters
the finishing station 317 which comprises two housing
parts 318, 319, in each of which can be found a pair of
finishing rollers biased towards an opposed cooling
roller carried on a fixed frame of the apparatus. Thus
the housing part 318 comprises finishing rollers 320,
322 biased towards an opposed cooling roller 301 to
form nips through which the web 12 extends. The first
housing part 318 is biased towards the fixed frame 328
of the apparatus (only partly shown) by springs 324.
Each finishing roller 320, 321 is provided respectively
with a radiant heater 330, 332, each with a steady
state output of 50 to 200 watts, to which electrical
power is controllably supplied by means not shown.
Each finishing roller 320, 321 is also provided with an
optional lubricating roller 340, 342 for applying a
release agent, such as silicone oil, to the surface of
the finishing rollers. The temperature of the surface
of the finishing rollers 320 and 322 is so controlled,
for example to 80 to 110°C and to 90 to 120°C
respectively, as to contact the surface of the web 12
at a temperature above the glass transition temperature
TQ of the toner to modify the finish of the toner
i i
i 2~a~ool
POSTGLOS - 44 - X1302B
image.
The web 12 leaving the first part of the finishing
station now passes over a second cooling roller 302,
whereby the opposite face of the web is cooled
sufficiently to prevent hot offset, for example so as
to again bring the temperature of the toner image on
the web below the glass transition temperature of the
toner, for example below 65°C. The cooling roller 302
is carried on the fixed frame of the apparatus opposite
to the second housing part 319 of the finishing station
317. Finishing rollers 321, 323 are carried in the
second housing part 319. The second housing part 319
is biased towards the fixed frame 328 of the apparatus
by springs 325, thereby biasing the finishing rollers
321, 323 into contact with the second cooling roller
302 to form nips through which the web 12 extends.
Each finishing roller 321, 323 is provided respectively
with a radiant heater 331, 333, each with a steady
state output of 50 to 200 watts, to which electrical
power is controllably supplied by means not shown.
Each finishing roller 321, 323 is also provided
respectively with an optional lubricating roller 341,
343 for applying a release agent, such as a silicone
oil, to the surface of the finishing rollers. The
283007
POSTGLOS - 45 - X1302B
temperature of the surface of the finishing rollers 321
and 323 is so controlled, for example to 80 to 110°C
and to 90 to 120°C respectively, as to contact the
surface of the web at a temperature above the glass
transition temperature Tg of the toner to modify the
finish of the toner image.
The cooling rollers 300, 301, 302 are hollow rollers,
fans 305, 303, 304 respectively being provided to blow
cold air through the interiors thereof.
The present invention is directed not only to printers
in which the toner image forming device is provided
with image information in an electronic form, but also
to devices in which the toner image forming device is
provided with image information in optical form, such
devices being referred to in the art as printers or
copiers depending upon their application. In the
context of the present invention, the term °printer° is
to be construed accordingly.
In the most preferred embodiment of the invention, the
fixing station uses a non-contact fixing device,
because this leads to a longer high quality lifetime
than contacting devices. The web of receiving material
is cooled after passing through the fixing station in
2183007
POSTGLOS - 46 - X1302B
order to strongly bond the toner to the receiving
material. In the finishing station where heat and
pressure are applied, only the top surface of the toner
image reaches the softening temperature, thereby
removing the peaks and filling the valleys in that
surface, leading to the desired more glossy appearance.
The heating of the toner surface in the finishing
station is insufficient to extend through the entire
bulk of the toner, so that weakening of the bond
between the toner and the receiving material, which
could lead to hot offset, is avoided. The heat
transfer in the finishing station is therefore less
than that in the fixing station. The rollers in the
finishing station are freely rotating rollers, driven
by movement of the web, although the use of directly
driven rollers is also possible.
