[0001] The invention relates to a method of treating image receiving sheets in a hot melt
ink jet printer, in which the sheets are advanced to a print surface where hot, molten
ink is applied onto the sheets, and to a printer employing this method.
[0002] US 6 196 672 B1 discloses a hot melt ink jet printer wherein, in order to accelerate the process
of heating the sheets to a temperature that assures a suitable solidification of the
ink, a preheater is provided which preferably has a higher surface temperature than
th print surface.
[0003] In printers in which paper sheets or similar image receiving sheets are used as recording
media, a tendency of the paper to cockle may sometimes constitute a serious problem.
The cockling phenomenon is related to the fact that paper and similar materials tend
to absorb humidity from ambient air and to expand and contract in accordance with
their humidity content. Typically, the expansion and contraction is unisotropic and
is particularly pronounced in a direction in which the fibers of the paper are predominantly
oriented. When there exists a gradient in humidity within the paper, then the more
humid portion of the paper will expand more than the drier portion, which inevitably
leads to the production of cockles or wrinkles.
[0004] In a typical setup of an ink jet printer, especially a large format printer, the
paper is intermittently advanced over a flat sheet support plate, while a carriage
moves back and forth across the paper, and ink jet printheads mounted on the carriage
are energized to eject droplets of ink onto the paper so as to form a printed image.
Since the carriage moves with relatively high velocity, the ink droplets ejected onto
the paper undergo a certain aberration and are deposited on the paper in a somewhat
dislocated position. The amount of dislocation is proportional to the flight distance
of the ink droplets. Thus, when cockles are present in the paper, the flight distance
is non-uniform and, accordingly, the dislocation of the spots of ink on the paper
also becomes non-uniform, so that the quality of the printed image is deteriorated.
[0005] In a hot melt ink jet printer, the ink is solid at room temperature and must be heated
above its melting point, typically in the order of magnitude of 100° C, before droplets
of liquid ink can be jetted onto the paper. As a result, when the image is being printed,
the paper will be heated by the high temperature of the ink, and part of the water
that has been absorbed in the paper will evaporate. This creates a humidity gradient
in the paper in the area of the print station, and the production of cockles is likely
to occur.
[0006] It is an object of the invention to provide a method and a hot melt ink jet printer
which are efficient in suppressing the cockling phenomenon.
[0007] According to the invention, the sheets, immediately before they reach the print surface,
are heated to a first temperature that is higher than a second temperature which they
will assume on the print surface.
[0008] The invention is based on the following consideration. When a paper sheet is kept
at a temperature that is significantly higher than the temperature the sheet used
to have before, the humidity that has been absorbed in the paper will gradually be
driven out, and the paper will shrink. However, the humidity content of the paper
does not decrease linearly but will rather decay with a certain time constant, pursuant
to a curve that resembles an exponential curve. Thus, initially, the humidity gradient
is steep, and it flattens more and more as time passes. At a higher temperature, the
time constant will be smaller and the humidity content will decay more rabidly. Since,
according to the invention, the sheet is heated to a relatively high temperature,
the humidity content moves down the steep initial portion of the decay curve within
a relatively short time, and when the paper reaches the print surface, the humidity
content is already on the flat "tail" of the curve, which will be even flatter because
the temperature on the print surface is lower. As a result, the time gradient of the
humidity content will be small when the paper is advanced over the print surface,
and, consequently, the spatial humidity gradient will also be small, so that the cockling
tendency of the sheet is reduced significantly.
[0009] More specific embodiments and further details of the invention are indicated in the
dependent claims.
[0010] The effect that has been described above will be more pronounced when the sheet is
heated to a higher temperature. On the other hand, the temperature should not become
too high in order to prevent damage to the paper. Moreover, an excessive initial temperature
of the paper would delay the solidification of the ink that is applied in the print
process. Therefore, preferably, the first temperature to which the sheet is pre-heated
should be 10 to 25 % higher than the second temperature on the print surface, if temperatures
are measured in centigrade. For example, if the average temperature of the paper on
the print surface is 32° C, then the paper should be pre-heated to a temperature of
about 38° C.
[0011] It is also preferable that the paper is exposed to the higher first temperature for
a relatively long time, which means that the pre-heating zone along the paper transport
path should be relatively long, of course without causing to much delay in the paper
feed process and without making the overall dimensions of the printer excessively
large. In a preferred embodiment, the paper is stored, e. g. in the form of an endless
web on a reel, in a paper magazine that is located below the print surface, and the
pre-heating zone is provided in an inclined portion of the paper feed path immediately
upstream of a feed roller which deflects the paper into a horizontal direction and
feeds it onto the print surface. This permits to provide a sufficiently long pre-heating
zone without substantially increasing the footprint of the printer and also permits
to arrange the heating zone in close proximity to the print surface, so that the paper
will not cool down again before it reaches the print surface. If the paper is supplied
in web form, the fibers of the paper are predominantly oriented in transverse direction
of the web, and consequently shrinkage will occur mainly in width direction of the
web. Thus, the cockles produced by a humidity gradient will extend in longitudinal
direction of the web. Then, the fact that the web is bent when it passes over the
feed roller helps to smoothen-out any cockles that may have been produced in the pre-heating
process.
[0012] Preferably, the printer comprises a humidity sensor arranged to detect the degree
of humidity of ambient air, and a control system adapted to control the pre-heating
process in response to the detected degree of humidity. When the air is relatively
dry, the cockling tendency is low, and the pre-heating temperature may be lowered
or the heat treatment may be dispensed with completely, in order so save energy. On
the other hand, when the humidity of ambient air increases, this is detected by the
humidity sensor, and the heater is automatically activated so as to mitigate the cockling
phenomenon and to assure a high print quality.
[0013] The pre-heating temperature may also be adapted to the type of paper that is being
processed. For example, when the paper magazine has a plurality of reels storing webs
with different widths or different paper qualities, the pre-heating temperature may
automatically be switched to the value that is most appropriate for the type of paper
that is being processed. Likewise, the active zone of the heater may automatically
be adapted in width to the width of the web, so that a waste of energy is avoided.
[0014] The pre-heating treatment may also be applied when processing a non-paper recording
medium, e. g. plastic film. In this case, the sheets will be less sensitive to humidity
but may have a larger thermal expansion coefficient, so that cockling may be caused
by a gradient in temperature. Then, the pre-heating temperature will be selected only
slightly above the temperature on the print surface, so that the temperature of the
sheet, when it reaches the print surface, will match the temperature of the print
surface itself.
[0015] A preferred embodiment of the invention will now be described in conjunction with
the drawings, in which:
- Fig. 1
- shows a schematic vertical cross-section of a paper transport system of a hot melt
ink jet printer according to the invention;
- Fig. 2
- is a schematic view of a paper sheet, illustrating the occurrence of cockles caused
by a humidity gradient in the paper; and
- Fig. 3
- is a diagram illustrating how the humidity content of the paper changes in the course
of time.
[0016] As is shown in Fig. 1, a hot melt ink jet printer comprises a frame 10 (which has
only been shown in phantom lines) and which accommodates a paper magazine 12 and a
paper feed system 14 adapted to feed a sheet 16 of paper to a print station 18 on
the top side of the frame 10. In the print station 18, the sheet 16 is sucked against
a flat top surface (print surface) of a perforated sheet support plate 20 by means
of a vacuum system (not shown). A carriage 22 is arranged to travel back and forth
across the sheet 16 in the direction normal to the plane of the drawing in Fig. 1
and carries at its bottom side a number of hot melt ink jet printheads 24 facing the
sheet 16. Thus, by energizing the printheads 24, a swath of an image is printed in
each pass of the carriage 22. Then, the sheet 16 is advanced by a step of appropriate
length in a direction indicated by an arrow A, so that the next swath can be printed.
A discharge mechanism 26 discharges the sheet onto a tray 28 which, in the example
shown, accommodates already a printed sheet 30. The sheet support plate 20 is temperature-controlled
in order to control the cooling rate and the solidification of the hot melt ink that
has been deposited on the paper. For example, the temperature of the print surface
of the sheet support plate 20 is kept at 32° C.
[0017] The paper magazine 12 comprises a set of six reels 32 each providing a supply of
printing paper in the form of an endless web 34. The reels 32 are arranged in three
levels, and the web 34 from each reel is drawn-off by means of a respectively associated
pair of transport rollers 36. An arrangement of guide plates 38 defines a branched
system of narrow feed paths 40 which merge into a common feed path 42 on the top side
of the paper magazine. The pairs of transport rollers 36 are selectively driven to
feed the web 34 from a selected one of the reels 32 to the common feed path 42. It
will be understood that the reels 32 may contain paper of different qualities and
possibly also non-paper recording media such as plastic films or the like. Further,
the webs on the reels 32 may differ in width, so that printed sheets may be produced
in different formats, ranging for example from A4 portrait to A0 landscape.
[0018] From the common feed path 42, the selected web is guided past a cutting mechanism
44 for cutting the web to the desired sheet length, and then the cut sheet 16 is guided
over a system of deflection and tensioning rollers 46 and guide plates 48 to a feed
roller 50 from which it is paid out onto the sheet support plate 20.
[0019] On its way from the reel 32 to the feed roller 50, the web 34 and the sheet 16, respectively,
will inevitably be exposed to ambient air and, as a result, will absorb humidity,
especially when the relative humidity RH of the ambient air is high. In the example
shown, the paper is particularly exposed to ambient air in the vicinity of the cutting
mechanism 44.
[0020] When the humidity content of the paper increases, it tends to expand, in particular
in the direction in which the fibers in the paper are predominantly oriented. Typically,
this is the direction transverse to the longitudinal direction of the web. When the
sheet 16, after having expanded in this way, reaches the sheet support plate 20 and
is heated by the hot melt ink deposited thereon, part of the water contained in the
paper will be evaporated, and the paper shrinks again in width direction of the sheet.
Thus, since a humidity gradient is present in the paper, the accompanying reduction
in width of the sheet leads to the production of cockles. This has exaggeratedly been
illustrated in Fig. 2, where a dashed line indicates an approximate border between
a more humid portion 52 and a drier portion 54 of the sheet 16. The shrinkage in width
of the drier portion 54 leads to the formation of cockles 56 in the portion 52.
[0021] In order to reduce the occurrence of such cockles 56 especially in the area of the
print station 18, a heater 60 is provided at an inclined portion of the paper feed
path just upstream of the feed roller 50, so that the humidity content of the sheet
16 is reduced already before it enters the print station. The heater 60 my for example
be formed by a heated plate along which the sheet 16 is guided. Since the sheet may
cockle when it passes along the heater, the heat transfer to the sheet is preferably
achieved not only by direct contact but also by heat radiation. As an alternative,
the heater may also be formed by a radiator of a hot air blower. The top surface of
the paper on the heater is preferably exposed to ambient air, so that the moisture
evaporating from the paper will efficiently be removed by convection.
[0022] The heater 60 is controlled by an electronic control system 62 which is also connected
to a humidity sensor 64. In the example shown, the sensor 64 is arranged in a position
where it can detect the humidity of air near a portion of the paper feed path where
the paper is particularly exposed to ambient air. Thus, when the air humidity is high,
the heater 60 is heated to a higher temperature in order to drive the humidity out
of the paper more efficiently. When the air humidity is lower, the temperature of
the heater 60 may be reduced, so that energy consumption is also reduced. Below a
certain threshold level of 40 % RH, for example, the heater 60 may be switched off
completely, because, then, the amount of cockling is within tolerable limits, anyway.
[0023] The control system 62 may also be programmed to establish a different relation between
the detected air humidity and the temperature of the heater 60, depending on the quality
(e.g. paper weight) of the print medium on the selected reel 32. For example, the
threshold for switching between an ON and an OFF state of the heater may be shifted.
[0024] In addition, the heater 60 may be segmented in width direction, an the width of the
active part of the heater will automatically be adapted to the width of the paper
web, so as to avoid a waste of heat energy.
[0025] When the humidity of ambient air is high, the heater 60 will heat the sheet 16 to
a temperature of 38° C for example. This temperature is higher than the temperature
of the print surface (32° C). The effect of this pre-heating strategy will now be
explained in conjunction with figure 3.
[0026] When a sheet 16 is intermittently fed towards the feed roller 50, the humidity content
of a certain area of the sheet which gradually travels along the heater will follow
a "decay" curve 66 shown in figure 3. This curve is initially relatively steep but
becomes flatter in the course of time. Would the sheet 16 be constantly kept at the
temperature of 38° C, the humidity content would follow the flat "tail" of the curve
66 that has been shown in dashed lines in figure 3.
[0027] However, at a certain point of time t1, the pertinent area on the sheet passes over
the feed roller 50 and reaches the print surface on the sheet support plate 20. The
temperature will drop while the sheet moves over the feed roller 50. In order to prevent
this temperature drop from becoming too large, the feed roller 50 may be made of a
material or may be coated with a material having a low heat conductivity. As the sheet
moves further over the print surface towards the printheads 24, its temperature will
gradually approach 32° C. The hot melt ink applied by the printheads 24 will transfer
a certain amount of heat onto the sheet, but this heat will rapidly be dissipated
through the temperature-controlled sheet support plate 20. As a result, as long as
the sheet passes over the sheet support plate 20, the temperature of the sheet will
be kept close to 32° C. This temperature corresponds to a slower decay of the humidity
content, as is indicated by the curve 68 in figure 3. It can be seen, that this curve
68 is relatively flat, so that the gradient in the humidity content is very small,
and, correspondingly, the sheet will hardly shrink further in width as it travels
over the sheet support plate. As a result, the production of cockles, especially in
the region of the print station 18, is successfully suppressed.
1. A method of treating image receiving sheets (16) in a hot melt ink jet printer, in
which the sheets are advanced to a print surface (20) where hot, molten ink is applied
onto the sheets, characterized in that the sheets, immediately before they reach the print surface (20), are heated to a
first temperature that is higher than a second temperature which they will assume
on the print surface.
2. The method according to claim 1, wherein the first temperature, when measured in centigrade,
is 10 to 25 % higher than the second temperature.
3. The method according to claim 1 or 2, wherein the print surface (20) is temperature-controlled
to keep the sheet (16) at said second temperature.
4. A hot melt ink jet printer having a print surface (20) and a sheet transport system
(14) for conveying image receiving sheets (16) to and over the print surface, characterized by a heater (60) arranged at a paper feed path upstream of the print surface (20), and
a control system (62) controlling the heater (60) and adapted to carry out the method
according to any of the claims 1 to 3.
5. The printer according to claim 4, wherein a humidity sensor (64) is arranged to detect
the degree of humidity of ambient air, and the control system (62) is adapted to control
the heater (60) in response to the detected degree of humidity.
6. The printer according to claim 5, wherein the control system (62) is adapted to switch
off the heater (60) when the detected degree of humidity is below a certain threshold
level.
7. The printer according to claim 5 or 6, wherein the control system (62) is adapted
to establish different relations between the detected degree of humidity and the activity
of the heater (60), depending on the type of printing paper being fed to the print
surface (20).
8. The printer according to any of the claims 4 to 7, wherein the paper transport system
(14) is adapted to supply the sheets (16) from a magazine (12), that is disposed underneath
the print surface (20), to a feed roller (50) situated at an upstream end of the print
surface (20), and the heater (60) is arranged at an inclined portion of the paper
feed path immediately upstream of the feed roller (50).
1. Verfahren zur Behandlung von Bildempfangsblättern (16) in einem mit heißschmelzender
Tinte arbeitenden Tintenstrahldrucker, bei dem die Blätter zu einer Druckfläche (20)
transportiert werden, wo heiße, geschmolzene Tinte auf die Blätter aufgebracht wird,
dadurch gekennzeichnet, dass die Blätter, unmittelbar bevor sie die Druckfläche (20) erreichen, auf eine erste
Temperatur aufgeheizt werden, die höher ist als eine zweite Temperatur, die sie auf
der Druckfläche annehmen werden.
2. Verfahren nach Anspruch 1, bei dem die erste Temperatur in Grad Celsius gemessen 10
bis 25% höher ist als die zweite Temperatur.
3. Verfahren nach Anspruch 1 oder 2, bei dem die Druckfläche (20) temperaturgeregelt
ist, um die Blätter (16) auf der zweiten Temperatur zu halten.
4. Tintenstrahldrucker für heißschmelzende Tinte, mit einer Druckfläche (20) und einem
Blatt-Transportsystem (14) zum Transportieren von Bildempfangsblättern (16) zu der
Druckfläche und über die Druckfläche hinweg, dadurch gekennzeichnet, dass eine Heizung (60) in einer Papiertransportbahn stromaufwärts der Druckfläche (20)
angeordnet ist und ein Steuersystem (62), das die Heizung (60) steuert, dazu eingerichtet
ist, das Verfahren nach einem der Ansprüche 1 bis 3 auszuführen.
5. Drucker nach Anspruch 4, bei dem ein Feuchtigkeitssensor (64) dazu angeordnet ist,
das Ausmaß der Feuchtigkeit der Umgebungsluft zu messen, und das Steuersystem (62)
dazu eingerichtet ist, die Heizung (60) in Abhängigkeit von der gemessenen Luftfeuchtigkeit
anzusteuern.
6. Drucker nach Anspruch 5, bei dem das Steuersystem (62) dazu eingerichtet ist, die
Heizung (60) abzuschalten, wenn die gemessene Luftfeuchtigkeit unterhalb eines bestimmten
Schwellenwertes liegt.
7. Drucker nach Anspruch 5 oder 6, bei dem das Steuersystem (62) dazu eingerichtet ist,
unterschiedliche Beziehungen zwischen dem gemessenen Ausmaß der Luftfeuchtigkeit und
der Aktivität der Heizung (60) herzustellen, je nach Art des Druckpapiers, das zu
der Druckfläche (20) zugeführt wird.
8. Drucker nach einem der Ansprüche 4 bis 7, bei dem das Papiertransportsystem (14) dazu
eingerichtet ist, Blätter (16) aus einem Magazin (12), das unterhalb der Druckfläche
(20) angeordnet ist, zu einer Zufuhrrolle (50) zuzuführen, die sich an einem stromaufwärtigen
Ende der Druckfläche (20) befindet, und die Heizung (60) in einem geneigten Abschnitt
der Papiertransportbahn unmittelbar stromaufwärts der Zufuhrrolle (50) angeordnet
ist.
1. Procédé de traitement de feuilles de réception d'images (16) dans une imprimante à
jet d'encre thermofusible, dans lequel les feuilles sont avancées jusqu'à une surface
d'impression (20) où une encre fondue et chaude est appliquée sur les feuilles, caractérisé en ce que les feuilles, immédiatement avant qu'elles atteignent la surface d'impression (20),
sont chauffées à une première température qui est plus élevée qu'une seconde température
qu'elles adopteront sur la surface d'impression.
2. Procédé selon la revendication 1, dans lequel la première température, lorsqu'elle
est mesurée en centigrades, est de 10 à 25 % plus élevée que la seconde température.
3. Procédé selon la revendication 1 ou 2, dans lequel la surface d'impression (20) est
régulée en température pour maintenir la feuille (16) à ladite seconde température.
4. Imprimante à jet d'encre thermofusible dotée d'une surface d'impression (20) et d'un
système de transport de feuilles (14) permettant d'acheminer des feuilles de réception
d'images (16) jusqu'à et sur la surface d'impression, caractérisée par un élément chauffant (60) agencé au niveau d'un chemin d'alimentation en papier en
amont de la surface d'impression (20), et un système de commande (62) commandant l'élément
chauffant (60) et adapté pour réaliser le procédé selon l'une quelconque des revendications
1 à 3.
5. Imprimante selon la revendication 4, dans laquelle un capteur d'humidité (64) est
agencé pour détecter le degré d'humidité de l'air ambiant, et le système de commande
(62) est adapté pour commander l'élément chauffant (60) en réponse au degré d'humidité
détecté.
6. Imprimante selon la revendication 5, dans laquelle le système de commande (62) est
adapté pour éteindre l'élément chauffant (60) lorsque le degré d'humidité détecté
est en dessous d'un certain niveau seuil.
7. Imprimante selon la revendication 5 ou 6, dans laquelle le système de commande (62)
est adapté pour établir différentes relations entre le degré d'humidité détecté et
l'activité de l'élément chauffant (60), selon le type de papier d'impression fourni
à la surface d'impression (20).
8. Imprimante selon l'une quelconque des revendications 4 à 7, dans laquelle le système
de transport de papier (14) est adapté pour fournir les feuilles (16) à partir d'un
magasin (12), qui est disposé sous la surface d'impression (20), jusqu'à un rouleau
d'alimentation (50) situé au niveau d'une extrémité amont de la surface d'impression
(20), et l'élément chauffant (60) est agencé au niveau d'une partie inclinée du chemin
d'alimentation en papier immédiatement en amont du rouleau d'impression (50).