FIELD OF THE INVENTION
The present invention relates to inkjet printing mechanisms,
such as printers or plotters.
- BACKGROUND OF THE INVENTION
More particularly the present invention relates to a mechanism
for cleaning a print head.
Nowadays inkjet printing systems are used in a wide array of
apparatuses in a wide array of applications such as fax, colour
photo printing, industrial applications etc. In these printing
systems inks, possibly of various colours, is ejected out of at
least one array of nozzles located in a print head to the receiving
A long known problem in inkjet printers is that the nozzles through
which the ink is projected to the receiving material are blocked by
clogging of ink inside the nozzles and on the print head. This
renders certain nozzles inoperable and results in a defective print
or deteriorated print quality.
To improve the clarity and contrast of the printed image, recent
research has been focused to improvement of the used inks. To
provide quicker, more waterfast printing with darker blacks and more
vivid colours, pigment based inks have been developed. These
pigment-based inks have a higher solid content than the earlier dye-based
inks. Both types of ink dry quickly, which allows inkjet
printing mechanisms to forms high quality images.
In some industrial applications, such as making of printing plates
using ink-jet processes, inks having special characteristics causing
specific problems. E.g. UV curable inks exist to allow rapid
hardening of inks after printing.
The combination of small nozzles and quick drying ink leaves the
print heads susceptible to clogging, not only from dried ink and
minute dust particles or paper fibres, but also from the solids
within the new ink themselves.
It is known to counteract or correct the problem of clogging by
protecting and cleaning the print head by various methods.
- Wiping : Before an during printing the inkjet print head is wiped
clean by using an elastomeric wiper, removing ink residue, paper
dust and other impurities.
- Capping : during non-operational periods the print head can sealed
off from contaminants by a sealing enclosure. This also prevents
the drying of the ink. The capping unit usually consists of a
rubber seal placed around the nozzle array.
- Spitting : by periodically firing a number of drops of ink through
each nozzle into a waste ink receiver, commonly called a spittoon,
clogs are cleared from the nozzles. This can be concentrated to
nozzles which are not used for a certain time but usually all the
nozzles are actuated during spitting.
- Vacuum assisted purging : During a special operation, in order to
clear partially or fully blocked nozzles, a printing is actuated
while on the outside of the nozzles a vacuum is applied. This
helps clearing and cleansing the nozzles. The purging is normally
performed when the print head is in the capping unit because this
unit can provide a good seal around the nozzle array for building
Also other methods exist for cleaning an inkjet print head which may
include applying solvents as in EP-A-1 018 430 ,
These features designed to clean and to protect a print head , are
commonly concentrated in a service station which is mounted within
the plotter chassis, whereby the print head can be moved over the
station for maintenance. An example of such a service station can
be found in US-A-6 193 353 combining wiping, capping, spitting and
It is inherent tot the purging action that a relatively large amount
of ink is used. The firing of the nozzles is usually done by
actuating all the nozzles at the same time but a sequential
actuation of the print elements can also be used.
- SUMMARY OF THE INVENTION
The ink can be drained from the capping enclosure by the same vacuum
source which provides the vacuum for purging. Also alternative
systems for removing the ink from the capping enclosure can be
After purging an excess amount of ink remains on the exterior side
of the print head, especially on the nozzle plate. In order to use
the print head for recording a wiping action is performed before
printing is started. A wiper passes along the printing surface of
the print head.
However several problems have been encountered during wiping after
purging and during wiping in general.
- Because the large amount of ink remaining on the outside of the
print head, the wiper has not enough capacity to clean the print
head in a satisfying manner. The large drops or blobs of ink can
not be removed sufficiently.
- A state of the art wiper consists of at least one elongated strip
of elastomeric material having the length of the nozzle array,
parallel with the array and is moved over the print head in a
directions perpendicular to the direction of the nozzle array.
Upon contact of the strip with the print head an considerable
force is exerted upon the print head. As the elastomeric material
will have a certain rigidity a mechanical shock will be given to
the print head as the whole length of the wiper simultaneously
makes contact with the print head. This shock can disturb the
normal equilibrium of the menisci in the nozzles of the print
head. Mechanical shocks can even cause intrusion of air into the
print head. Also the mounting means of the print head and the
wiper can be affected by the shock.
- During the wiping action the whole length of the wiper is slightly
deformed as it is held in contact with the print head. The total
force acting upon the print head and wiper system as they are kept
in pressure contact with each other during the wiping action puts
a lot of strain on the mounting assembly and moving mechanism of
the print head and wiper system.
- At the end of the wiping action the elastomeric strip reaches the
end of the print head and suddenly recoils to its original
position. This also generates a mechanical shock while at the same
time the ink residues on the tip of the wiper, which are
especially large during the first wiping step after purging, are
flung away contaminating the inside of the printer.
The result is that after purging normal wiping is insufficient while
the wiping action using state of the art wipers gives rise to
considerable mechanical stress leading to alignment errors and
contamination of the printer.
Even after wiping a large amount of ink can remain on the side of
the print head forming a meniscus.
It is clear that several drawbacks have to be overcome.
- BRIEF DESCRIPTION OF THE DRAWINGS
The above-mentioned advantageous effects are realised by a method
having the specific features set out in claim 1. Specific features
for preferred embodiments of the invention are set out in the
Further advantages and embodiments of the present invention will
become apparent from the following description and drawings.
- DETAILED DESCRIPTION OF THE INVENTION
- Fig. 1A and 1B show a transversal and lateral view of a print head
with accumulated ink drops.
- Fig. 2 illustrates the skimming action.
- Fig. 3 shows the drainage of the excess of ink along the skimmer.
- Fig. 4 shows a dual wiping system in contact with the print head.
- Fig. 5 illustrates the angled positioning of the wiper system to the
- Fig. 6 shows a wiper having bevelled edges.
The drawbacks are avoided by an improved method for cleaning the
The drawbacks are avoided by wiping the print head using a wiper
having a slanted position. The direction of the wiper and the
direction of the nozzle array are not parallel.
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.
After purging large drops and blobs of ink which have accumulated on
the print head as shown in Fig. 1A and 1B. These are removed by a
skimmer slowly skimming the print head thereby making contact with
drops of ink on the surface of the print head, but without making
contact with the surface of the print head itself.
The skimming action is illustrated in Fig. 2. The distance d
between the print head and the skimmer needs to be smaller than the
size of the drops hanging from the surface of the print head. As
illustrated in Fig. 3, when the skimmer makes contact with a drop on
the surface it will, due to surface tension characteristics and the
effect of gravity, be drained downwards along the skimmer.
This provides a pre-cleaning step after purging, avoiding problems
caused by the excess of ink during the wiping action following the
The distance d between the bottom side of the print head and the
skimmer is in the range of 0-5mm. Skimming alternatively can be
performed by just touching the print head without exerting force to
the print head. Preferably a clearance is kept between the print
head and the skimmer. Preferably the minimum distance at which the
skimmer passes the print head is between 0.5 and 5mm.
The direction in which the skimmer passes along the print head is
not important but preferably a mechanism is provided which is
coupled to the mechanism for moving the wiper. Then a skimmer having
the size of the print head is used. Fig. 2 shows the situation
wherein the skimming direction is perpendicular to the direction on
the nozzle array.
In Fig 3. the skimmer has an angled position to the direction of the
nozzle array. Direction of movement is not critical.
Possibly the skimming movement is along the direction of the nozzle
array. A small skimmer can then be used having a size of the
broadness of the nozzle array.
Preferably the wiper itself serves as skimmer. This is most cost
effective as no separate system has to be provided for skimming. By
lowering the wiper so that is just clears the print head during
wiping causes it to skim the surface of the print head to serve as
skimmer for removing excess of ink. During skimming the speed of
translation of the wiper is reduced as to provide enough time to
allow the drop to drain slowly to the bottom as can be seen in
Fig 3. No separate translation mechanism is needed, only the speed
is to be adapted. Skimming speeds are preferably between 0.001 and
A advantageous effect is that due to the liquid ink, having a higher
solvent content than dried ink, the wiper is partially cleaned by
ink running along the surface of the wiper.
Distance d of the skimmer or wiper from the print head and speed
during sweeping can be chosen arbitrarily. Depending upon
characteristics of the ink in use and its surface tension, the
property op the outer surface of the inkjet print head,
constitution, size of the wiper, etc... Skimming distance d and
speed can be chosen to obtain optimal result, if possible in
combination with the subsequent wiping action.
Possible skimming can be performed using a wick for draining ink
from the print head.
Also a foil can be used for skimming the print head.
Normally the skimmer is an elastomeric element having a hardness of
30 to 80 Shore A.
The skimming action is followed by a normal wiping action. This can
be done using a single or a double wiper as shown in Fig. 4. During
wiping the wiper is pressed against the print head.
- Practical example
In order to avoid mechanical shocks, stress and undesirable flinging
of ink from the wiper during recoil, the wiper is placed, as
illustrated in Fig. 5 for a dual wiper system, at a small angle α
regarding the direction perpendicular to the translation direction.
The angle α can be any angle between 1 and 99 degrees but the range
is practically between 1 and 10 degrees. Preferably the angle α is
in the range between 1 and 3 degrees.
As seen in Fig 5 the wiping direction is usually perpendicular to
the direction of the nozzle array this means that the wiper blades
also are at a small angle α to the nozzle array.
- During wiping the wiper makes a first contact with the print head
at the first end. Because only a small part of the wiper
encounters the print head no great mechanical forces are involved.
- During further movement of the wiper the contact point gradually
moves from the first end to the trailing end. Parts of the wiper
clearing the print head will not recoil but fold back slowly as
the neighbouring part of the wiper is still in contact with the
print head during the wiping action.
- When the trailing end of the wiper clears the print head, this end
will show a certain amount of recoil, but due to the restricted
length of wiper blade involved the forces are small and no ink
will be flung from the wiper resulting in less contamination of
the printing mechanism. This can even be improved by bevelling the
extremities of the wiper blades as indicated in fig. 6. The
bevelled edges are preferably located outside the area of the
nozzle array on the print head.
It is clear that less mechanical stress and shocks cause less wear
and tear in the printer.
The wiper preferably has at least the length of the nozzle array.
Wiping direction may vary. Usually wiping will be done in a
direction perpendicular to the nozzle array, but other directions
can be possible. Wiping can be done perpendicular to the direction
of the slanted direction of the wiper itself. Alternatively a
smaller wiper can be used wiping in the direction of the nozzle
Preferably the wiper has a hardness between 30 and 80 ShoreA.
An inkjet printer having a shuttling print head having a length of
72mm is used for printing images using an oil based ink.
The inkjet print head has a dual array of nozzles having a length of
The print head is capped by the capping unit having a seal around
the nozzle array.
This is done by placing the print head over the capping unit and
raising the maintenance station so the capping unit now holds the
print head. A vacuum is applied inside the capping unit and at the
same time the nozzles are driven in order to clear clogged nozzles.
Released ink is drained from the capping unit.
After purging the print head, the maintenance unit is lowered so the
print head clears the capping unit. As a result of purging large
drops of ink are formed hanging down from the print head. Depending
of the volume of the drops they typically can have dimensions of 1
to 5mm in height.
The print head is now brought near the wiper system mounted on the
maintenance station. The wiper has at least a length of the print
head which is about 54mm. The maintenance station is raised so that
the edge of the wiper is brought to a height corresponding to 0.5mm
clearance with the bottom edge of the print head.
The wiper is used as skimmer by translating it underneath the print
head at a speed of 0.02m/s. Drops hanging from the print head are
thus captured and flow downwards along the wiper.
After the skimming action the wiper is repositioned and is further
raised and a wiping action is started.
The wiper has a hardness of 70 ShoreA and the direction of the wiper
makes an angle of 2 degrees relatively to the direction of the
nozzle array. Wiping is performed at a speed between 0.1 to
0.5m/sec. Preferably the speed is about 0.2m/sec.
Wiping speed can be adjusted according to ink and wiper
characteristics, e.g. surface tension, density of the ink, hardness
or length of the wiper etc... An empirical approach can be used
determining the optical speed, constitution and pressure
force/contact angle of the wiper.
The skimming step is hereinbefore described as a treatment of the
print head after purging. It can be understood that the skimming
step can be performed at any moment if necessary during the printing
The wiping action can also be performed at any moment if the need
exists. Also a periodical wiping can be provided. It is clear that
the slanted position of the wiper relative to the nozzle array is
also advantageous during each wiping action.
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.