The invention relates to a method of printing a substrate
using an inkjet printer, which printer comprises a holder to rotatably receive a
roll on which the substrate is wound, a downstream print zone and an inkjet printhead
for printing the substrate at the print zone, a transport means for engaging and
transporting the substrate to the print zone during which transport the substrate
is unwound from the roll, and a guide element which is situated downstream of the
roll in front of the transport means to guide the substrate from the roll to the
transport means, the method comprising: transporting the substrate over a predetermined
distance with control of the transport means, during which transport the guide element
is moved from a first position which said element occupies prior to the transport,
to a second position such that the distance over which the substrate extends between
the roll and the transport means is smaller as a result of the movement and after
the substrate has been transported over the predetermined distance, printing a strip
of the substrate with control of the inkjet printhead, and after printing of the
strip the re-transport of the substrate over a predetermined distance during which
the guide element is moved, which transport is followed by printing a following
strip of the substrate.
This method is used inter alia to prevent damage to the
substrate during its transport. The transport means, which is frequently a transport
nip which engages the substrate at a number of places distributed over the width
of the substrate, has a lower mass inertia than the roll on which the substrate
is wound, at least when there is a specific minimum amount of substrate present
on the roll. The transport of the substrate and its simultaneous unwinding from
the roll can in that case take place at adequate speed if the transport means is,
for example, driven by a very powerful motor, both the transport means and the roll
being accelerated. The result of such a powerful motor is that considerable forces
are exerted on the substrate, with the risk that the substrate will tear. In addition,
a powerful motor of this kind has the disadvantage that it is less suitable for
very accurate control. In inkjet printers in particular, accurate control of the
transport means is very important, because printing often takes place by printing
the substrate in a number of swaths, and in each swath part of the substrate is
printed, often a strip of the same width as the inkjet printhead. All the sub-images
together form the image for printing. For accurate juxtaposition of the sub-images,
accurate transport of the substrate is desirable.
It has been chosen to provide the transport means with
a low-power driving motor and to dispose a movable guide element between the roll
and the transport means. By moving this element, it is possible to reduce the distance
over which the substrate extends between the roll and the transport means. This
prevents any sudden increase in the substrate tension. The motor that drives the
transport means consequently does not have to move the entire roll each time. It
is thus possible to use a relatively low power drive for the transport means, it
being a relatively simple matter to make such drive accurate. In this method, the
roll can be driven separately with a powerful but less accurate motor.
A disadvantage of this method is that for accurate transport
of the substrate by the transport means there is required not only an accurate drive
but also a very accurate transport means and guide element. The mechanical tolerances
such as roundness, straightness, mutual parallelism, etc, are subject to 'stringent
requirements in order that transport may take place with the required accuracy.
In addition, even with very close tolerances, there are still a relatively large
number of irregularly recurring juxtaposition faults between the sub-images. These
juxtaposition faults are indications of random faults in the substrate transport.
The object of the invention is to obviate the above disadvantages.
To this end, a method according to the preamble has been invented which is characterised
in that the guide element is brought into the first position prior to the re-transport
of the substrate.
Thus whenever the substrate is moved an increment for printing
of a following strip, the guide element is first brought into the same initial position.
This is possible if the substrate, during each transport increment, is transported
by the transport means the same distance as said substrate is unwound from the roll.
Since the mass inertia of the roll differs from that of the transport means, the
transport of the substrate by the transport means and the unwinding of the substrate
from the roll are out of phase, but this difference can be compensated by the movement
of the guide element.
It has been found that the application of this method results
in far fewer irregularly recurring juxtaposition faults between the sub-images.
Regularly recurring juxtaposition faults still occur, but this problem can be obviated
easily by calibration. As is sufficiently known from the prior art, regularly recurring
faults can easily be taken into account, for example during control of the inkjet
printhead or heads, so that these faults can be completely eliminated.
Since the guide element is in each case brought to the
initial position, the method of transport including all faults therein which occur
due to out of true, crookedness, non-parallelism, friction, slip, compliance, etc,
is practically the same each time and there is therefore a pattern of regular deviations
in the transport. Since they are known in advance, it is possible easily to compensate
for these regularly recurring faults. Thus it is possible to use transport means
and guide elements with less accurate mechanical tolerances.
In one embodiment, in order to transport the substrate
to the print zone, a part of the substrate is unwound from the roll first prior
to control of the transport means for the purpose of such transport. In this embodiment,
therefore, the method starts first with unwinding part of the substrate from the
roll. This gives a kind of loop of "free" substrate. This loop can be restricted
to a minimum, for example, by moving the guide element. The transport medium itself
is not actuated until some time later. Directly prior to this, care is taken to
ensure that the guide element is in its fixed initial position. Since the roll has
already been unwound one increment, there will be no shortage of "free" substrate
even in the event of considerable acceleration of the transport means. As soon as
there is a risk of excessive tension in the substrate as a result of further transport
thereof, this tension can be reduced by moving the guide element in the direction
of the second position. After the transport of the substrate is complete, the guide
element is returned to the first position, and if necessary a corresponding amount
of substrate is unwound from the roll for the purpose by driving the drive motor
for the roll.
In another embodiment, the maximum speed at which the substrate
is unwound from the roll during transport of the substrate is less than the maximum
speed of transport imparted by the transport means. This embodiment has the advantage
that the roll, which may have a relatively high mass inertia, does not acquire any
very high speed of revolution. That could in fact cause problems during the stoppage
of the roll. Rapid braking of an inert roll may cause shocks and hence have a negative
effect on the accuracy of the substrate transport. In addition, the requirements
to be met by the drive motor for the roll are reduced because as a result of the
lower maximum speed it is possible to apply reduced accelerations.
The invention also relates to an inkjet printer for printing
a substrate, provided with a holder to rotatably receive a roll on which the substrate
is wound, a downstream print zone and an inkjet printhead for printing the substrate
at the print zone, a transport means for engaging and transporting the substrate
to the print zone, and a guide element which is situated downstream of the roll
in front of the transport means to guide the substrate from the roll to the transport
means, wherein the guide element is disposed movably in the printer in such manner
that it can be moved between a first position in which the distance over which the
substrate extends between the roll and the transport means has a first value, and
a second position in which said distance has a second value less than the first,
characterised in that the printer is provided with a control unit which ensures
that the guide element occupies the first position directly prior to the transport
of the substrate over a predetermined distance having as its objective to make a
strip of the substrate available for printing.
In one embodiment this printer is provided with a spring
element which provides resistance to displacement of the guide element. By the use
of a spring element it is possible to obtain a relatively low resistance to movement
of the guide element. This can further minimise any build-up of tension in the substrate.
In another embodiment, the spring is disposed parallel
to the guide element. It has been found in this way that it is very easy to provide
adequate resistance to movement of the guide element, said resistance being sufficiently
low not to have an adverse effect on the substrate transport. In yet another embodiment
the spring consists of a number of weak springs coupled in series.
In one embodiment, upstream of the guide element the printer
has a second transport means for transporting the substrate. The advantage of this
embodiment is that there is a further decoupling of forces between the drive for
the first transport means and the drive for the roll. Thus the substrate can be
kept taut between the first and second transport means despite the fact that a section
of substrate has already been unwound from the roll prior to transport by the first
transport means. This gives more possibilities of accurate transport of the substrate.
In one embodiment, the guide element is a roller which
comprises a shaft on which a number of substantially congruent (identical in shape)
wheels are disposed. In yet another embodiment, the roller is provided with a curved
guide plate to guide the substrate to the roller. This has been found advantageous
during introduction of the web over the transport means, and can have a positive
effect on further preventing unwanted mechanical wear of the substrate during its
The invention will now be explained in detail with reference
to the following examples.
- Fig. 1 is a diagram of a printer according to a specific embodiment of the present
- Fig. 2 shows a guide element that can be used as a guide for the substrate.
- Fig. 3 shows another embodiment of a guide element.
- Fig. 4 is a diagram showing the speeds at which the substrate is transported
through the transport nips 32 (Fig. 4A) and 31 (Fig. 4B).
Fig. 1 is a diagram of a printer according to the present
invention. This printer is provided with the supply unit 10, which serves for the
storage and delivery of the substrate for printing. In addition, this printer comprises
a transport unit 30 which transports the substrate from the supply unit 10 to the
print engine 40. Unit 30 also provides accurate positioning of the substrate in
the print zone formed between the print surface 42 and the inkjet printhead 41.
In this embodiment, print engine 40 is a conventional engine which comprises printhead
41, which printhead is constructed from a number of separate sub-heads, each of
one of the colours: black, cyan, magenta and yellow. Printhead 41 has only a limited
printing range so that it is necessary to print the image on the substrate in different
sub-images. To this end, the substrate is transported an increment in each case
so that a new part of the substrate can be printed in the print zone. In the example
illustrated, the substrate 12 comes from a roll 11 from the supply unit 10. A web
of the substrate is wound on this roll, the web having a length of 200 metres. To
accommodate the roll in the printer, the supply unit is provided with a holder (not
shown) to receive the roll rotatably. This holder consists of two parts mounted
in side plates of the printer, which parts are brought into co-operative connection
with the ends of the roll. In this embodiment, the supply unit is provided with
a second holder to receive roll 21. Another substrate 22 is wound on this roll and
can also be delivered by the supply unit for printing. For the transport of the
substrate, roll 11 is operatively connected to transport means 15, which means in
this case comprises a pair of rolls between which a transport nip is formed. More
particularly, means 15 relates to a set of two shafts each extending in a direction
substantially parallel to roll 11, on which shafts a number of roll pairs are mounted
each forming a transport nip for the substrate. In an alternative embodiment, only
one roll pair is mounted on the shafts, substantially coinciding with the middle
of the web 12.
Upstream of means 15 is a sensor 17, by means of which
it is possible to determine whether there is still substrate on the roll situated
in the associated holder. As soon as the roll is used up, the end of the web will
pass the sensor, and this is detected by the sensor. For the transport of a substrate
originating from roll 21, the supply holder is provided with transport means 25.
Upstream of this means the supply holder is provided with sensor 27, which has the
same action as sensor 17. The supply holder is provided with guide elements 16 and
26 to guide the substrates 12 and 22 respectively to the transport unit 30. Downstream
of these guide elements, there is a transit path 13. This transit path is used both
for the transport of substrate 12 and the transport of substrate 22.
A substrate leaving the supply unit 10, in this example
substrate 12, is engaged by transport means 31 of the transport unit 30. This transport
means transports the substrate via guide element 33 on to the second transport means
32 of the transport unit 30. The transport means 32 engages the substrate, transports
it to print engine 40 and ensures good positioning of the substrate in the print
zone between the print surface 42 and the printhead 41. The transport means 31 and
32 extend substantially parallel to the rolls 11 and 21, and have a length such
that the substrate can be engaged over substantially its entire width.
The guide elements 16 and 26 are in this example rollers
extending parallel to the transport means 15 and 31; 25 and 31 respectively. They
are substantially stationary rollers, i.e. they cannot rotate about their axial
axis. For the substrate 12 illustrated, this means that during transport the substrate
slides over element 16 and is at the same time fed in the direction of transport
means 31. When this configuration is used it has been found that movement of the
substrate at the guide element in a direction parallel to the direction in which
the element extends is possible. In other words, the substrate can in this way make
a lateral movement with respect to the direction in which said substrate is transported.
The reason that a lateral movement of this kind is possible in this configuration
is associated with the fact that the substrate makes a sliding movement with respect
to the guide element. As a result, the required frictional force to set the substrate
in motion initially with respect to the guide element is already overcome and practically
no force is needed to move the substrate laterally over the guide element.
The guide elements are so disposed in the supply unit that
they can each rotate, at least through a limited angle, about an axis substantially
perpendicular to the direction in which said guide elements extend (i.e. the axial
direction of the guide elements). In the Figure, the rotational axis 18 of element
16 is shown, and also rotational axis 28 of element 26. These rotational axes are
perpendicular to the axes of the guide elements and intersect the centre of said
elements. As a result of this rotation combined with the possibility of moving the
substrate laterally, the substrate has been found to have very good guidance from
the supply unit 10 to nip 31 of the transport unit 30. As a result, despite the
fact that the transport means 15 and 31; 25 and 31 respectively are not perfectly
parallel, it is nevertheless possible to transport the substrate without any damage
Guide element 33 of transport unit 30, which element extends
substantially parallel to the transport means 31 and 32, is also so disposed that
it can rotate about an axis perpendicular to the axial direction of said element.
This axis is shown by reference 34 and intersects the centre of guide element 33.
Since element 33 in this embodiment is a co-rotating roller, the substrate is substantially
stationary with respect to the surface of said guide element. As a result, a lateral
movement of the said substrate at said element is made difficult. In order that
such a movement can be made possible, element 33 is so suspended that it can rotate
about axis 35, which axis 35 extends parallel to the bisector 36 of the angle 2&agr;
over which the substrate is fed from means 31 to means 32. This axis 35 intersects
the centre of the substrate web at a distance of about 1 metre from the guide element
itself. On rotation of element 33 about this axis, the substrate makes a substantially
lateral movement. The possibility of rotation of element 33 over the axes of 34
and 35 ensures flexible and accurate transport of the substrate from transport means
31 to transport means 32, even though the two means do not extend 100% parallel
to one another.
Guide element 33 is movable from a first position in which
said element is situated in Fig. 1, to a second position in which the centre of
this element coincides with the location 37. In the first position, the distance
over which substrate 12 extends between transport means 31 and transport means 32
is at a maximum. In the second position this distance is at a minimum. Use is made
of this fact during the transport of the substrate to print engine 40. Since the
substrate must in each case be moved over a relatively short distance, typically
5 to 10 cm, it is advantageous for this to occur relatively quickly. The mass inertia
of roll 11, certainly when it is provided with the maximum quantity of substrate,
is relatively high however. For this reason, if the configuration of transport means
and guide elements as illustrated were maintained, movement would take relatively
considerable time. To counteract this problem, transport means 31 is accelerated
much more slowly than transport means 32. In order nevertheless to ensure adequate
supply of substrate to transport means 32, the guide element 33 is moved in the
direction of location 37. As a result, there is no lack of substrate at transport
means 32 during its passage to print engine 40. If the passage by means 32 is stopped,
the residue at transport means 31 is compensated by allowing the said transport
means to continue rotating for some time. In these conditions, the element 33 is
moved back to the first position. In this way, prior to a following transport of
a part of the substrate requiring printing with print engine 40, guide element 33
is in the same initial starting position. It has been found that in this way very
accurate transport of the substrate is possible. As a result, the various sub-images
can match up more satisfactorily and the number of print artefacts can be reduced.
The provision of accurate transport and particularly accurate
positioning of the substrate in the print zone by control of means 32, is related
to the fact that the substrate is engaged by both means 31 and means 32. The position
of the substrate is more satisfactorily defined as a result. Together with the rotational
possibilities of guide element 33, in this way very accurate transport and positioning
of the substrate is obtained, the tension in the substrate not increasing to an
extent such that under normal circumstances mechanical damage of the substrate would
occur. An important additional advantage of this arrangement is that printing can
still be continued on the substrate as long as the end of the web has not passed
transport means 31. The instant at which this happens can easily be determined if
the end of the web is detected by means of the sensor 17 or 27 corresponding to
this web. It is then a simple matter to determine what length of the substrate can
still be fed on to the print engine 40 before said end of the web passes the means
31. In this way it is possible to determine whether the image printed at that instant
can still be completely imaged on the substrate without the end of the web passing
the first transport means. If so, that image will be completed. If not, then it
is possible to choose to stop printing. However, when the end of the web passes
means 31 the transport and the positioning of the substrate may be accompanied by
more errors, and this may result in print artefacts. Too many artefacts can result
in the image having to be reprinted. In order to save ink and substrate it is therefore
better to stop printing.
If it is still possible to print the current image on the
substrate (without the end of the web passing the means 31), it is then possible
to determine whether the next image for printing can still be printed on the substrate
(without the end of the web passing the means 31). If so, that image will be printed.
If not, then it is better to print this following image on a new substrate, for
example originating from roll 21.
Fig. 2 shows a guide element 116 which can be used in a
preferred embodiment as a guide for the substrate in the supply unit 10 (instead
of the guide element 16 and/or 26). Fig. 2A is a side elevation of this element.
This element comprises a bent plate comprising a part 200 situated upstream of the
bend 202, and a part 201 which is situated downstream of the bend 202. Part 200
is connected by spot welds 206 to the rigid frame part 205. The frame part 205 is
a U-profile extending over the length of element 116 and connected to the frame
of the printer. Part 201 of the plate is much less restricted in its freedom of
movement than part 202. Yoke 210 fixed on the U-profile 205 on its own provides
a point of support for part 201, and in this connection see the front elevation
of element 116 as shown in Fig. 2B. It will be clear from this front elevation that
part 201 is substantially free. Since the plate is relatively thin, part 201 is
torsionally weak and can at least partially rotate about the axis passing through
the centre of the yoke 210 and perpendicular to the longitudinal axis of element
116. In one embodiment, part 201 is provided with slots so that this part has less
resistance to torsion.
If element 116 is placed in the supply unit to replace
element 116, the free end of plate part 200 points towards the transport nip 15
and part 201 is practically parallel to transit path 13 of the supply unit. Element
116 is also stationary in the supply unit. As a result of the tension in the substrate
part 201 can be pulled against yoke 210. As a result, the ends particularly of part
201 can rotate about the axis passing through the centre of the yoke, perpendicularly
to the direction in which element 116 extends. The advantages of this rotational
possibility are described under Fig. 1.
Fig. 3 is a diagram of one embodiment of guide element
33. In this embodiment element 33 comprises a shaft 300 on which a series of transport
wheels 301 are disposed. The substrate is guided over these wheels. Since the shaft
is suspended to be freely rotatable, it can co-rotate with the substrate without
any mutual difference in speeds. As a result, the frictional force accompanying
the transport of the substrate at the roller is practically only dependent on the
friction in the mounting of this roller.
Element 33 is provided with a guide plate 302 bent in the
form of a V to assist in guiding the substrate. It should also be clear that the
V-shape of the element 302 substantially coincides with the V-shape of the substrate
as shown in Fig. 1. Shaft 300 is resiliently suspended by leaf springs 305 and 306
which are fixed to be freely rotatable on fixed frame parts 307 and 308 respectively.
These leaf springs each form the same angle with the shaft in such manner that the
centre lines of the leaf springs have a point of intersection 310 upstream of the
roller. Rotational axis 35 intersects this point of intersection. Fig. 3B shows
the suspension of the shaft in greater detail. The leaf spring 305 is fixed on the
end of shaft 300. Leaf spring 305 is in turn fixed on shaft 311 which is suspended
to be freely rotatable in U-shaped frame part 307. By means of this suspension it
is possible for roller 33 to rotate about the axes 34 and 35. Although the rotational
possibility is finite, it appears to be sufficient to make possible accurate and
reliable transport of the substrate between the nips 31 and 32.
Fig. 3C diagrammatically shows the spring mechanism with
which roller 33 is pushed in the indicated direction A. This direction A coincides
with the direction extending from the above-mentioned second position that the element
33 can occupy (see Fig. 1, location 37) to the first position that the element occupies
in Fig. 1. To this end, the shaft 300 is provided with side panels 315 and 316 which
at their end remote from the shaft are provided with elements 317 and 318 respectively.
The set of weak springs 322, 323 and 324 is fixed on these elements, this set being
guided over freely rotatable wheels 320 and 321. The springs are to some extent
stretched so that they tend to move the ends of the set of springs to the centre
thereof, as indicated in Fig. 3C. As a result, the elements 317 and 318, and hence
the shaft 300, are pushed in the indicated direction A.
Since the construction chosen results in a resistance to
the displacement of the roller, a stiffness in respect of movement of translation
is introduced for the roller in principle. During movement of the roller to the
second position, the resistance to this movement becomes increasingly greater. The
advantage of this resistance is that the movement of the roller takes place more
accurately and more satisfactorily reproducibly. By placing a number of long weak
springs in series, this resistance remains sufficiently small but very effective.
Fig. 4 diagrammatically shows the speeds at which the substrate
is transported through the transport nips 32 (Fig. 4A) and 31 (Fig. 4B) during the
passage of part of said substrate so that a new strip thereof can be printed using
inkjet printhead 41.
Curve 400 in Fig. 4A shows what speed of passage is imposed
on the substrate at the nip 32. A high speed of transit is generated relatively
quickly and this is retained for some time and then drops to zero rapidly. Despite
the high mass inertia of the roll on which the substrate is wound, this high acceleration
can be obtained by moving roller 33 as indicated under Fig. 1.
Curve 401 in Fig. 4B shows the speed of transit imposed
on the substrate at nip 31 for the transport of the same length of the substrate.
It will be seen that this nip is driven before nip 32 so that the substrate is already
partly unwound from roll 11 before nip 32 is driven. It may happen that movement
of the roller 33 will enable the web to be tensioned between the means 31 and 32.
The acceleration which is imparted by nip 31 is smaller than that of nip 32, and
the maximum speed of transit that this nip provides is lower. However, the substrate
is passed through for a longer time so that ultimately the same length of the substrate
passes the nip 31.