FIELD OF THE INVENTION
[0001] The present invention relates to digital printing apparatus for borderless printing
and mesh printing.
More specifically the invention is related to a reconfigurable print table for an
inkjet printer.
BACKGROUND OF THE INVENTION
[0002] Printing is one of the most popular ways of conveying information to members of the
general public. Digital printing using dot matrix printers allows rapid printing of
text and graphics stored on computing devices such as personal computers. These printing
methods allow rapid conversion of ideas and concepts to printed product at an economic
price without time consuming and specialised production of intermediate printing plates
such as lithographic plates. The development of digital printing methods has made
printing an economic reality for the average person even in the home environment.
Conventional methods of dot matrix printing often involve the use of a printing head,
e.g. an ink jet printing head, with a plurality of marking elements, e.g. ink jet
nozzles. The marking elements transfer a marking material, e.g. ink or resin, from
the printing head to a printing medium, e.g. paper or plastic. The printing may be
monochrome, e.g. black, or multi-coloured, e.g. full colour printing using a CMY (cyan,
magenta, yellow, black = a process black made up of a combination of C, M, Y), a CMYK
(cyan, magenta, yellow, black), or a specialised colour scheme, (e.g. CMYK plus one
or more additional spot or specialised colours). To print a printing medium such as
paper or plastic, the marking elements are used or "fired" in a specific order while
the printing medium is moved relative to the printing head. Each time a marking element
is fired, marking material, e.g. ink, is transferred to the printing medium by a method
depending on the printing technology used. Typically, in one form of printer, the
head will be moved relative to the printing medium to produce a so-called raster line
which extends in a first direction, e.g. across a page. The first direction is sometimes
called the "fast scan" direction. A raster line comprises a series of dots delivered
onto the printing medium by the marking elements of the printing head. The printing
medium is moved, usually intermittently, in a second direction perpendicular to the
first direction. The second direction is often called the slow scan direction.
[0003] The combination of printing raster lines and moving the printing medium relative
to the printing head results in a series of parallel raster lines which are usually
closely spaced. Seen from a distance, the human eye perceives a complete image and
does not resolve the image into individual dots provided these dots are close enough
together. Closely spaced dots of different colours are not distinguishable individually
but give the impression of colours determined by the amount or intensity of the three
colours cyan, magenta and yellow which have been applied.
[0004] In order to improve the veracity of printing, e.g. of a straight line, it is preferred
if the distance between dots of the dot matrix is small, that is the printing has
a high resolution. Although it cannot be said that high resolution always means good
printing, it is true that a minimum resolution is necessary for high quality printing.
A small dot spacing in the slow scan direction means a small distance between marker
elements on the head, whereas regularly spaced dots at a small distance in the fast
scan direction places constraints on the quality of the drives used to move the printing
head relative to the printing medium in the fast scan direction.
[0005] Generally, there is a mechanism for positioning a marker element in a proper location
over the printing medium before it is fired. Usually, such a drive mechanism is controlled
by a microprocessor, a programmable digital device such as a PAL, a PLA, a FPGA or
similar although the skilled person will appreciate that anything controlled by software
can also be controlled by dedicated hardware and that software is only one implementation
strategy.
One general problem of dot matrix printing is the formation of artefacts caused by
the digital nature of the image representation and the use of equally spaced dots.
Certain artefacts such as Moiré patterns may be generated due to the fact that the
printing attempts to portray a continuous image by a matrix or pattern of (almost)
equally spaced dots. One source of artefacts can be errors in the placing of dots
caused by a variety of manufacturing defects such as the location of the marker elements
in the head or systematic errors in the movement of the printing head relative to
the printing medium. In particular, if one marking element is misplaced or its firing
direction deviates from the intended direction, the resulting printing will show a
defect which can run throughout the print. A variation in drop velocity will also
cause artefacts when the printing head is moving, as time of flight of the drop will
vary with variation in the velocity. Similarly, a systematic error in the drive system
for moving the printing medium may result in defects that may be visible. For example,
slip between the drive for the printing medium and the printing medium itself will
introduce errors.
[0006] Especially in large size inkjet printers and industrial inkjet printing machines,
the receiving medium transport system has to be very accurate and reliable in transport
distance to avoid banding problems.
These systems usually must be capable to handle different sizes and thickness of receiving
media.
Another problem is that the printing speed and transport speed is much higher than
those of office or home inkjet printers.
These industrial printers often use a web-based material as printing stock. The web
based material has to be fed very correctly as small deviations would lead to skew
feeding of the web which could lead to malfunctioning of the printer. Small feeding
deviations in sheet-fed material do not pose such a problem as each sheet is independently
taken from the paper bin, unless sheet-fed material is pre-printed and is to be accurately
aligned in the printer to register the image to be printed to the already pre-printed
image.
[0007] A problem also encountered is that printing on large size rigid media poses specific
problems in respect to positioning and transporting of the media.
- Rigid media normally have a greater weight than paper and have greater inertia than
light materials which poses greater needs on the media transport system.
- Due to the rigidity it is also possible that the material can not be straightened
out easily and due to unevenness of the material surface the throw distance may
vary and certain printing defects can occur.
- Certain rigid materials exhibit a certain porosity so that they can not be easily
transported by a transport system using vacuum forces to hold a medium. This problem
is very apparent when one wants to print on mesh material, rigid or flexible.
[0008] Another aspect in industrial printers is that the shuttle containing the printheads
is usually relatively heavy in comparison to home or office printers. Due to the higher
shuttle speed, the drops follow a sloping path from the printhead to the receiver.
Even the slightest deviation in throw distance between the head and the receiver will
result in deviations in positioning the ink drops.
The throw distance has to be kept constant over the full width of the shuttle and
over the full length of the shuttle movement.
[0009] It has been shown that transport rollers do not provide a solution to the problems
described. Another drawback is that when using large size receiving media rollers
are needed in the middle of the receiving medium and that these rollers come into
contact with the fresh printed surface.
[0010] In
WO 01/56 804 a conveyance apparatus is provided for stepwise conveying of materials which can
be used in an inkjet printer.
The apparatus uses fixed and moving elements for holding the working portion of the
material, being the portion of the conveyed material on which the tool, in this case
the inkjet printhead, is working on. The apparatus of
WO 01/56 804 has however certain drawbacks.
- Support of the working portion of the receiving medium is always divided over several
elements of which some completely static and some are movable for transporting the
receiving medium. The support structure is formed by the movable and fixed elements
Therefor it can not be assured that the material is supported over the whole width
at the same height and with the same force. Especially when printing thin, flexible
media this would lead to problems.
- As the moving elements are in contact with the receiving medium at the printing location
no movement of these elements is tolerated during printing as longitudinal forces
would be exerted upon the receiving medium at the printing location. This inevitably
leads to a slower feeding speed.
- The apparatus is not able to transport materials having high porosity and mesh-like
materials which are not laminated to a liner fabric.
- the vacuum transport elements support only about 50% of the width of the material
which gives possibly not enough force to move the heavier or porous materials.
[0011] It is clear that there is still a need for improvement of these transport systems.
It is the aim of the invention to provide a receiving media transport system that
can handle all types and sizes of receiving media having a very exact positioning
capability.
SUMMARY OF THE INVENTION
[0012] The above-mentioned advantageous effects are realised by a method of reconfiguring
the print table as set out in the specific features set out in claim 1. Specific features
for preferred embodiments of the invention are set out in the dependent claims.
[0013] Further advantages and embodiments of the present invention will become apparent
from the following description and drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014]
- Fig. 1
- gives a schematic overview of the media transport system according to the invention.
- Fig. 2
- depicts the principle of segmented vacuum chambers and using blind valves.
- Fig 3A
- to 3F gives the different positions of the tables and vacuum applied during different
transport of the medium and the printing step.
- Fig. 4
- gives an embodiment using toothed vacuum tables.
- Fig. 5
- shows a vacuum table having a bevelled edge to avoid paper block during sheet feeding.
- Fig. 6
- Depicts a possible embodiment of a media holding assistance system.
- Fig 7.
- Shows the replacement of some removable static table sections for border-less printing.
- Fig. 8
- shows the replacement of removable static table sections by a gutter for mesh printing.
DETAILED DESCRIPTION OF THE INVENTION
[0015] The solution to the problems is provided by method using a reconfiguration of the
print table
[0016] While the present invention will hereinafter be described in connection with preferred
embodiments thereof, it will be understood that it is not intended to limit the invention
to those embodiments.
General description
[0017] Media transport systems as depicted in Fig 1 normally also comprises
- The printing unit with the step-wise media transport system.
- A feeding roll to deliver non-printed receiving medium to the printing unit.
- A take-up roll for storing the printed medium. Alternatively it is possible to deliver
the material to a finishing unit to cut the material at appropriate length eventually
followed by further finishing such as folding, stapling, etc...
[0018] Other embodiments of the printing apparatus may comprise a sheet feeder and alignment
unit in front of the printing unit having the step-wise media transport system, and
a sheet lay off and stacker unit to receive the printed sheets. This embodiment may
be used for flexible sheets as well as rigid materials.
These elements are however not show for clarity.
[0019] The media transport system.
There is provided a static table 1 that holds the media during a printing action when
the inkjet-printing head 2 performs a fast scan along a guidance 3 over the receiving
media as a swath is printed. During the printing action the whole working part 4 of
the receiving medium is substantially supported by the static table 1. This means
that the static table 1 has at least the width and the length to support the area
of the receiving material on which the tool will operate, in this case an inkjet printhead
2 will record a swath of the image.
As shown in the embodiment of Fig 1 two dynamic tables 5 and 6 are present for holding
and transporting the media during a transport step, but it would be possible to use
only one table if the material has a certain stiffness or can be maintained in a fixed
position while the one dynamic table repositions underneath the material.
The transport steps are performed in between printing steps, by using a step and repeat
mechanism described in more detail further on. The receiving medium is therefore always
static during printing and a high accuracy in feeding the receiving medium in distance
and orientation can be obtained leading to less artefacts in the printed image.
The forces for holding the receiving medium can be any sort of force but is preferable
capable of being switched. The forces could be electrostatic, magnetic (certain media)
or preferably vacuum.
[0020] Fig. 1 gives an overall view of the medium transport system according to the present
invention using vacuum forces to hold the receiving medium.
Static vacuum table
[0021] Central to the system is static vacuum table 1 that holds the receiving medium static
during the printing action.
The top surface is formed by a rigidly fixed plate having small perforations 7 of
about 0.5 to 2mm wide to enable the vacuum to attract the receiving medium lying above
it during the printing action. Also small grooves (about 0.5mm)are provided to distribute
the vacuum over a lager area.
The perforations can also be replaced by small slits in the top plate.
[0022] Preferable the plate is page-wide provided at the working area 4 which is the actual
area printed by the inkjet printhead 2 during a fast scan print action. The aim is
to thoroughly support the receiving material over the total width of the working area
4. Especially when using thin media this is important. No moving parts of the medium
transport system are located under the working area 4. Only fixed parts are present
under the working area 4.
Under the perforated plate there is provided a vacuum chamber 8 in connection with
the perforations 7. Table 1 and vacuum chamber 8 form a closed box in which a vacuum
can be created. Vacuum is applied and maintained by an air evacuation system, e.g.
a ventilator system, drawing air out of the vacuum chamber 8 to obtain a vacuum in
the chamber.
The air evacuation system has enough capacity to generate sufficient vacuum in a short
time to that the receiving medium can be immobilised on the vacuum table 1 quickly.
Plural vacuum chambers
[0023] If the width of the receiving medium is less than the with of the plate, the problem
rises that, through the perforations 7 which are not covered by the receiving sheet,
air will flow into the chamber 8 and the vacuum cannot be maintained easily or is
partially lost.
A solution to this problem, of which a possible solution is illustrated in Fig 2 is
that instead of a single vacuum chamber 8, the plate surface is divided into several
fields each having their own vacuum chamber 8. Especially when the dimensions of these
fields are chosen and designed in relation to common paper widths it is always possible
to obtain a good vacuum to rigidly hold the receiving sheet in place. Vacuum chambers
8 outside the width of the receiving sheet may lose vacuum or may be switched of from
the vacuum source, but have no influence on the holding power of those chambers 8
underneath the receiving sheet.
Vacuum release valve
[0024] When the receiving sheet should be released, vacuum should be discontinued in the
chamber(s). This can be done by stopping the air evacuation means, but preferably
a valve 9 is provided in one of the walls 10 of the vacuum chamber. The valve 9 is
opened and air is let into the chamber 8 or between chambers 8. The cross-section
of the valve 9 is preferably large and especially a blind 11 valve can be employed
as they tend to have a large opening and they can be switched very quickly between
open and closed state. Vacuum can be switched without even turning the air evacuation
means off.
Dynamic vacuum tables
[0025] Dynamic vacuum tables 5 and 6 provide the moving part of the media transport system.
These are designed to hold the receiving layer during incremental transport steps
of the receiving medium and may release the receiving layer once held by the vacuum
of the static table 1.
A dynamic vacuum table 5,6 is provided at each side of the static vacuum table 1.
The top surface is formed by a plate having small perforations 7 to enable the vacuum
to attract the receiving medium lying above it during the transport action. Also here
slits can be provided
Over at least a certain length of the receiving medium the plate is provided page-wide
to keep the transport forces constant over the width of the receiving medium.
Under the perforated plate there is also a vacuum chamber 8 in connection with the
perforations. Vacuum is created and maintained by an air evacuation system.
The air evacuation system has enough capacity to generate sufficient vacuum in a short
time to that the receiving medium can be drawn to the dynamic vacuum table quickly.
Plural vacuum chambers
[0026] Likewise as in the static vacuum table 1, if the width of the receiving medium is
less than the width of the plate, the problem rises that through the perforations
7 which are not covered by the receiving sheet air will flow into the chamber 8 and
the vacuum cannot be maintained easily or is partially lost.
A solution to this problem is that instead of a single vacuum chamber 8, the plate
surface is divided into several fields each having their own vacuum chamber 8. Especially
when the dimensions of these fields are chosen and designed in relation to common
paper widths it is always possible to obtain a good vacuum to rigidly hold the receiving
sheet in place. Vacuum chambers 8 outside the width of the receiving sheet lose vacuum
or are switched of from the vacuum source, but have no influence on the holding power
of the other chambers 8 underneath the receiving sheet.
[0027] As also in the static vacuum table 1, blind valves 9 form an excellent method of
switching the state of the vacuum table between holding and releasing state.
As the dynamic vacuum tables 5,6 move, they preferably are of a lightweight construction
that gives less inertia problems at the start and end of the transport step.
[0028] As it is the intention that both dynamic vacuum tables 5,6 move synchronously during
transport of the receiving medium they are preferably relatively mounted fixed to
each other.
[0029] In figure 1 both dynamic vacuum tables 5,6 are driven by common spindles 12 so they
always move at the same speed. Alternatively they can be rigidly coupled to each other
to form one unit which is driven by a single spindle system 12.
At both edges of the dynamic vacuum tables a guide rail (not shown) is provided for
guiding the moving tables along a correct path.
[0030] Both spindles 12 may be driven by high resolution step motors 13 to have accurate
control over the length of the transport step and speed.
This can eliminate the need for encoders to determine exact position and speed of
the dynamic tables 5,6.
Temperature considerations
[0031] Due to the large width of the vacuum tables in certain industrial printing machines
the thermal expansion of the tables 5,6 can not be discarded.
At one side the guide rail and spindle system 12 over which the dynamic tables 5,6
move is rigidly fixed. The opposite rail can be mounted in a floating way to allow
for the expansion of the tables 5,6. Using fixed rails at both sides would result
in stress causing deformation of the dynamic vacuum tables 5,6 and less accurate transport
of the receiving medium. A possible embodiment is given in fig 1 using sliding mountings
14.
Also for the static vacuum table 1 it has to be avoided that stress will occur due
to thermal expansion and possibly lead to deformation of the table 1.
Method of operation
[0032] The operation of the media transport system is a step-wise incremental transport.
[0033] During the printing action of the apparatus a web or sheet material is provided.
- 1. As the swath of the image is printed, the medium can be fe forward.
- First, if not already done, the vacuum in the dynamic tables 5,6 is build-up by closing
the blind valves 9 in the vacuum chambers 8 of the dynamic tables 5, 6. As the vacuum
is generated the receiving medium 15 is drawn into contact by the force of the vacuum
acting upon the receiving medium 15.
- As the receiving medium 15 is fixed unto the dynamic tables 5,6 the blind valves 9
of the static vacuum table 1 are opened to release the grip of the static vacuum table
on the receiving medium 15. As the vacuum is lost the receiving medium 15 is only
attached to the dynamic vacuum tables5,5. Fig 3A
- 2. During the transport step Fig 3B the dynamic tables 5,6 are set into movement by
starting the step motors 13 to turn the spindles 12 driving the dynamic vacuum tables
5,6, located at both end of the dynamic vacuum tables 5,6. The speed of the two spindles
12 has to be kept the same to ensure parallel feeding of the receiving medium 15.
This can be done by exact control of the two step motors 13 driving the spindles 12.
As both dynamic vacuum tables 5,6 are closely coupled by a rigid coupling or by the
spindles 12 at both sides, they automatically have the same speed. In an alternative
embodiment both vacuum tables 5,6 could have different driving mechanisms but this
poses even more problems in speed control of the motors.
In an alternative embodiment the step motors 13 and spindles 12 for driving the dynamic
vacuum tables 5,6 can be replaced by linear motors. Since the medium 15 transport
is an incremental stepping transport with a short stroke travel distance, linear motors
may be very well suited for this job.
During movement the dynamic vacuum table unit is moved in a downstream direction,
i.e. dynamic table 6 is moved closer to or into contact with static vacuum table 1
while table 5 is pulled away from static vacuum table 1.
During movement the receiving medium 15 is translated together with the dynamic vacuum
tables 5,6 unit to which it is adhered.
The distance over which the step-wise translation is done can be controlled by the
step motors 13 or by using a separate detection means such as an appropriate encoder.
It has been shown that an accuracy of 3 µm could be obtained using step motors, which
is quite sufficient for inkjet recording systems.
- 3. After translation the dynamic vacuum table unit is in its downstream position (Fig
3C) and the vacuum in the static vacuum table 1 is again established by closing the
blind valves 9 and as the receiving medium 15 is adhered to the vacuum table 1 the
next recording step can begin.
Vacuum of the dynamic vacuum tables 5,6 is removed by opening the blind valves 9 in
the dynamic vacuum chambers 8 (Fig. 3D)
The transport module is put back into starting position (upstream position) by reverse
rotation of the step motors 13. The dynamic vacuum table unit may be brought to the
starting position (Fig 3E) at a time outside the printing step as to avoid disturbing
the printing. To speed up printing it is possible to perform the backwards step of
the dynamic tables 5,6 during the printing by the shuttling printhead, but care has
to be taken that the printing process is not disturbed by the mechanical movement
of the dynamic tables 5,6.
- 4. Before printing a swath of the image, the working area of the receiving media 15
is adhered to the static vacuum table 1. This is done by closing the blind valves
9 of the vacuum table 1, i.e. of the different vacuum chambers 8 that are covered
by the receiving medium 15, so that vacuum can be build up inside these vacuum chambers
8.
As the vacuum builds up inside the vacuum chambers 8 the medium 15 is drawn into contact
with the perforated base plate and is held in place by the force of the vacuum.
In Fig 3F The dynamic vacuum table unit, comprising dynamic tables 5 and 6 at either
side of the static table 1, is located in an upstream position relative to static
tabel 1, i.e dynamic table 6 is located more upstream (further away) from static table
1 than dynamic table 5 is located downstream from static table 1. Dynamic table 5
is located close to or in contact with the static table 1. The movement of tables
5 and 6 is stopped. Possibly the vacuum in the dynamic vacuum tables 5,6 is also activated
to fix the receiving medium 15 even more rigidly. After the working area 4 of the
receiving medium 15 is held on the base plate of the static table 1 it is possible
to reliably print a swath of the image by shuttling the inkjet printhead 2. The distance
between the printhead 2 and the receiving material 15 is at the desired value as the
base plate of the static table 1 is present over the whole length and width of the
working area 4.
The printhead 4 performs a fast scan over the receiving medium 15 along a guidance
3 and prints a swath of the image to be recorded. This can be done in a single pass
over the working area 4 (unidirectional printing) or by a dual pass as the printhead
2 shuttles over the working area 4 and is returned to the start position and a partial
image is printed each time (bidirectional printing).
ALTERNATIVE EMBODIMENTS AND VARIATIONS
[0034] The interface between the static table 1 and the dynamic tables 5,6 can be a straight
boundary, but in an alternative embodiment the tables 1,5,6 can fit to each other
using a toothed pattern as shown in Fig. 4. However it is important that the whole
working area 4 of the receiving medium 15 is substantially supported by the static
vacuum table 1.
[0035] The invention can be used for the step-wise transport of a web material to be printed
on, but likewise is would be possible to transport sheet material using the system.
An improvement shown in Figure 5 that could be used in sheet feeding is that the upstream
edges of one or more vacuum tables 1,5,6 is bevelled to avoid that the leading edge
of a sheet hits the upstream edge of the table and a deviation would occur in feeding
the sheet.
[0036] During printing of an image on the receiving medium each time a step having a certain
step distance is made. However the step distance can be variable as this can be necessary
in certain recording methods.
MEDIA HOLDING ASSISTANCE
[0037] When the printing medium 15 can not be adhered properly to the dynamic tables 5,6
by the vacuum, possibly due to porosity in the case of mesh-media, a combination of
unevenness or rigidity in the case of rigid media, or any other reason, it is possible
to provide the vacuum tables 5,6 with a additional clamps or force system to assist
the holding of the media 15 during printing and transport. This media holding assistance
system may contain rollers (either full width rollers extending across the full width
of the media or a number of smaller rollers spread along the full width of the media),
fingers or styli, clamps, suction cups, etc. The assistance system may be mounted
upstream or downstream of the working area where the printing occurs, or at both sides
of the working area. In a preferred embodiment, as shown in fig 6, the media holding
assistance may have a set of styli 16 that can push the receiving medium 15 against
the static 1 and/or dynamic table 5,6, to prevent receiving medium 15 from sliding
away from these tables. More preferably the system may have two rows of styli 16,
one row for pushing the receiving medium against the dynamic table 5,6 and the other
row for pushing the receiving medium against the static table 1. Care must be taken
that the styli 16 located above the static table 1 do not interfere with the working
area 4 of the medium 15 where the printhead 2 is moved back and forth across the medium
in the a fast scan direction for printing a swath of the image. If the table 1 is
wide enough in the direction of receiving medium transport, the styli 16 can be placed
just before and/or after the working area 4. If the static table 1 is to narrow, the
styli 16 for assisting the vacuum table 1 in holding the receiving medium 15 during
printing may be placed outside, i.e. upstream or downstream the static/dynamic table
assembly, i.e. on a frame part 17 of the printing apparatus where the receiving medium
15 slides over. This configuration is illustrated in figure 6. As discussed above,
the static 1 and dynamic 5,6 tables work in harmony with each other in a repetitive
cycle of holding the receiving medium 15, e.g. the dynamic table 5,6 holding the medium
15 while moving the dynamic table 5,6 downstream, and releasing the receiving medium
15, e.g. the dynamic table 5,6 releasing the medium 15 while moving the dynamic table
5,6 upstream again. The styli 16 from the media holding assistance system may be activated
simultaneously with the activation of the vacuum on the dynamic or static vacuum table,
in which case the assisting styli 16 operate in the same repetitive cycle as the vacuum
of the tables 1,5,6, but other activation schemes are perfectly possible. The styli
16 may be activated by pressed air and approach the receiving medium 15 from above
pushing it against the supporting table 1,5,6 or frame part 17 underneath the styli
16. In a preferred embodiment, the amount and location of the sytli 16 is chosen so
as to have an equal assistance of the receiving medium 15 transport over the full
width of dynamic/static table or frame part in a direction perpendicular to the medium
transport direction. Alternative embodiments for the styli 16 may be roller (operation
from above the receiving medium 15), suction cups (operating from underneath the receiving
medium 15 and assisting to the small vacuum holes in the tables) or any other suitable
means. The styli, rollers, suction cups, etc. may be resiliently mounted so as to
not damage the receiving medium 15 on impact.
When a media holding assistance system is provided at the downstream side of the working
area of printing, care must be taken the assistance means do not damage the image
that was just previously printed. This may be the case in printing systems using inks
that take time to dry. Sometimes it may be sufficient to have active drying means
in or near the working area 4, e.g. on the shuttle together with the printhead 2,
so that the printed pixels or swaths are at least "touch dry" when leaving the working
area 4 and entering the area of the downstream dynamic table and/or media holding
assistance system.
[0038] The static/dynamic table assembly and vacuum support may also be assisted by roller
pairs known from web transport and web tensioning systems. The roller pairs can hold
the receiving medium in a fixed and tensioned state during printing wherein the vacuum
of the static/dynamic table assembly is for holding the receiving medium flat, and
forward the receiving medium in the transport direction in between the fast scans
for printing a swath of the image. The roller pairs are preferably tension controlled
and limited with a maximum torque to avoid slip of the receiving medium over the vacuum
tables, i.e. to avoid that the tension of the roller pairs onto the receiving medium
exceeds the holding force of the vacuum tables. Roller pair embodiments may include
two independent rollers, one upstream and another downstream the working area of the
receiving medium, operation against a sliding or rolling contact area on the printer
frame or dynamic tables. The receiving medium moves between a roller and a part of
the printer frame or dynamic table. Alternatively the embodiment may include two roller
pairs, one upstream and another downstream the working area of the receiving medium.
The receiving medium then passes in the nip of the rollers of each of the roller pairs.
When mesh-media or rigid media are used, onto which the holding force of the vacuum
tables is low, the roller pairs may be the major means for forwarding and tensioning
(if applicable) the receiving medium. In other words, the static/dynamic vacuum tables'
functionality in the media transport is mainly to support the mesh and rigid media
during transport.
[0039] It has to be noted that it is not necessary that the dynamic vacuum tables need to
be lowered when moving upstream underneath the web.
[0040] The stepping motors 13 can be directly coupled to the spindle drives 12 or they can
be coupled using a gearing system. All depends upon the type of step-motor 13, spindle
12 and desired accuracy and speed of the movement. The two spindles need to operate
exactly at the same speed, so preferably high quality motors are used which are coupled
to each other by electronic gearing.
[0041] Another important aspect is that the inkjet printhead needs to be at a constant distance
from the receiving medium. As the printhead shuttles, it can be understood that an
ink drop also follows a sloped path in its way to the receiving layer. Any distance
variation will therefor result in a dislocation of the ink dot in the fast-scan direction.
Distance variation can be caused by a variation in height of the printhead.
[0042] The present transport system is capable to transport a web as shown directly from
the feeding roll, although, dependent upon the type of medium to be fed, it may be
advantageous to avoid tension on the receiving medium at the print location by providing
a separate web feed module unrolling the feeding roll and buffering a lot of the feeding
roll tension. This can provide even more accurate feeding. It has been found a significant
advantage that the operation of the static/dynamic table media transport generates
no shear forces in the receiving medium and that the receiving medium is in a "tensionless
state" during printing.
PRINTING ON MESH-MEDIA / BORDER-LESS PRINTING
[0043] When printing onto mesh-media 18 that is not supported on a liner material, it is
inevitable that printing ink or other marking material that is used for printing,
is deposited through the mesh of the media, straight onto the static table 1. During
the subsequent sliding transport of the media over the static table, the ink or marking
material deposited on the static table is smudged on the back the mesh-media. A similar
problem occurs when border-less printing onto receiving media 15 is targeted, e.g.
poster or photo printing. Printing up to the border without going over the border
is a feature of a printing apparatus that is very hard to realise.
A solution is given by a method according to the the present invention including the
step in which the printing table is reconfigured. A solution to the problems of smudging
ink or marking material on the static table 1 is provided by a static table 1 that
is segmented along the length of the table, i.e. the dimension along the fast scan
direction, in a number of removable sections 19. As illustrated in figures 7 and 8,
as a result of the reconfiguration step, the removable sections may be replaced by
bucket sections 20 or a single full-length bucket 21 may be provided standard underneath
the full-length of the static table. These removable sections may be used in two different
configurations:
- Before border-less printing on an ink or marking material impermeable receiving medium
15, the table sections 19 that are not fully supporting the receiving medium are removed
and depending on the embodiment replaced with bucket sections 20. This results in
a static table 1 that is, in operation, fully covered by the receiving media 15. The
ink or marking material that is possibly printed outside of the receiving medium 15
area is collected in the inserted bucket sections 20 or the standard full-width bucket
21.
- When mesh-media 18 is used, all sections 19 of the static table 1 are removed in the
reconfiguration step and replaced by bucket sections 20 if applicable. During printing
the mesh-media 19 will be still supported and maintained in a fixed position by the
application of the vacuum of the dynamic tables 5,6 possibly assisted by the media
holding assistance means described above.
[0044] An even more preferred embodiment of the removable static table sections 19 allows
maximum support of the receiving medium 15 by not removing the whole of the static
vacuum table section 19 but limiting the area that is removable from the static vacuum
table section to the working area 4 of the printhead 2 or the shuttle, i.e. the area
where ink or other marking material may be deposited. If the static table 1 is wider,
along the direction perpendicular to the fast scan direction, than the width of a
print swath, then only the area of the static table sections 19 corresponding with
the area 4 of a print swath are removed or replaced with buckets 20. The remaining
part of the static table sections 19 that are not corresponding with a print swath
remain in place and may keep on supporting the receiving media 15 during printing
of a print swath. In this preferred embodiment the static vacuum table 1 is divided
into sections along the length of the table and each section is again divided into
the width direction into a working area part and a support part.
[0045] It may be advantageous if the vacuum table sections 19, working area parts and bucket
sections 20 are individually mountable as inserts onto the vacuum chambers 8 underneath
the static vacuum table 1. In this case, replacing table sections etc. does not involve
changing the vacuum chamber configuration underneath the table.
VERTICALLY ADJUSTABLE STATIC TABLE
[0046] It has been said before that the distance between the marking tool, e.g. the ink
jet printhead 2, and the receiving medium 15 must be very well controlled to have
an optimal functioning digital printing process. When rigid media 15 are used, the
flatness of the media 15 itself will be of major importance. When flexible media 15
are used, the flatness of the static table 1 on which the flexible media is pulled
via the vacuum will be of major importance. The static vacuum table 1 is therefore
adjustable in height at multiple locations so that it can conform to the height profile
of the shuttle or printhead along the fast scan direction. In a preferred embodiment,
the static table 1 may be divided into multiple sections 19 along the fast scan direction.
These sections may individually be controlled at different heights. This provided
optimum calibration of the distance between the marking tool 2 and the receiving medium
15, along successive sections of the fast scan movement. Height adjustment of the
static table sections 19 may be realised by one or more height adjustment screws per
section, or any other means known in the art for adjusting the height of the table
sections 19. If multiple adjustment screws per table section 19 are used, not only
the average height of the table section 19 but also the inclination of that table
section 19 may be adjusted. In a preferred embodiment the static table sections 19
may have a dimension, along the fast scan direction, in a range of a couple of cm
up to tens of cm, depending on the targeted or required accuracy of the distance marking
tool 2 to receiving medium 15.
Having described in detail preferred embodiments of the current invention, it will
now be apparent to those skilled in the art that numerous modifications can be made
therein without departing from the scope of the invention as defined in the appending
claims.