Ink jet printing apparatus and method providing an induced, clean-air region

To protect critical operative portions (e.g. the orifice plate) of ink jet printing apparatus from airborne debris particles (e.g. paper dust), a wall is provided to substantially enclose a region around those critical operative portions and filtered air is induced to flow through the enclosed region from a location proximate the operative portions to a droplet outlet. In one mode the energy of printing droplet streams induces the air flow. In another embodiment the energy of printing substrate movement induces the air flow.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to ink jet printing apparatus and more 
specifically to improved constructions and procedures in such apparatus 
that effect an air flow, for protecting the print head assembly from 
contamination, that is induced by an operative printing function(s) of the 
apparatus. 
2. Description of the Prior Art 
The term "continuous" has been used in the field of ink jet printer 
apparatus to characterize the types of ink jet printers that utilize 
continuous streams of ink droplets, e.g. in distinction to the "drop on 
demand" types. Continuous ink jet printers can be of the binary type 
(having "catch" and "print" trajectories for droplets of the continuous 
streams) and of the multi-deflection type (having a plurality of print 
trajectories for droplets of the continuous streams). Binary type 
apparatus most often employs a plurality of droplet streams while 
multi-deflection apparatus most often employs a single droplet stream. 
In general, the print head assembly of continuous ink jet printing 
apparatus includes an ink cavity to which ink is supplied under pressure 
so as to issue in a stream from an orifice plate in liquid communication 
the cavity. Periodic perturbations are imposed on the liquid stream (e.g. 
vibrations by an electro-mechanical transducer) to cause the stream to 
break up into uniformly sized and shaped droplets. A charge plate is 
located proximate the droplet breakoff point and imparts electrical charge 
in accord with a print information signal to effect selective droplet 
deflection in accord with the charge borne by the droplet. A catcher is 
provided to catch non-printing droplets. 
Certain elements of such a print head assembly, e.g. the orifice and charge 
plates, should be of relatively minute scale so as to provide good 
printing resolution. These elements must be fabricated and mounted very 
precisely in order to achieve quality printing. Thus dust particles are 
repugnate to the nature of the print head assembly; and a serious problem 
is presented by large volumes of print media moving in very close 
operative relation with the print head assembly. In other ink jet printing 
apparatus, e.g. drop on demand printers, it is desirable to protect at 
least the orifice structure from dust particles. 
The most successful prior art approaches to this problem involve forcing 
clean (e.g. filtered) air into the region of the critical print head 
elements to prevent unwanted dust and debris from reaching those elements. 
However, the prior art approaches for providing the protective air flow 
have certain disadvantages. First, the external blowers used to provide 
such a protective air flow add cost, size, energy usage and noise to the 
printing apparatus. When the print head assembly is a moving part, it is 
difficult to maintain a uniform air flow from the blowers to the print 
head assembly; and it is highly desirable, from the viewpoint of accurate 
droplet placement, that the air flow conditions around the droplet path be 
quite stable. That is, ink droplets are very small and influenced in 
trajectory by low-velocity air currents so that uniform air flow, in a 
direction generally parallel to the droplet flight path is highly desired 
for optimum print quality. 
SUMMARY OF THE INVENTION 
The purpose of this invention is to solve the above-described problems 
connected with dust or debris in ways that avoid the disadvantages of 
prior art approaches and achieve high printing quality. Thus one 
significant objective of the present invention is to provide, in ink jet 
printing apparatus, improved structure for preventing paper dust and other 
such debris from reaching critical zones within the apparatus print head 
assembly. Another objective is to provide a protected droplet flight zone 
which is free from debris and non-stable air currents that adversely 
affect droplet trajectory. 
These objects are achieved in accordance with one embodiment of the present 
invention by providing in ink jet printing apparatus of the type having 
(i) a print head assembly, including an orifice plate for directing 
droplets toward a print substrate and (ii) means for providing relative 
movement between the print head assembly and a print substrate, an 
improved protection structure comprising wall means that substantially 
encloses a region around tne orifice plate from external air and has a 
droplet outlet, an air inlet passage into the region enclosed by such wall 
means and means for filtering air flowing into the enclosed region through 
the air inlet passage. 
By virtue of such structure, and in accord with the procedures of the 
present invention, the printing movements of ink droplets and/or relative 
print-head/print-medium movement induces external air to flow through said 
filtered inlet passage, into the enclosed region and out of the droplet 
stream outlet in a stable manner. In one preferred embodiment, continuous 
droplet streams provide the predominant inducing energy for such 
protective air flow. In another preferred embodiment, the movement of 
print substrate provides the predominant energy for inducing such 
protective air flow. In other preferred embodiments the droplet stream and 
substrate movement energies can be utilized to effect protective air flow. 
Certain embodiments of the present invention are constructed to provide 
induced air flow that protects lower print head structures (e.g. a charge 
plate and/or a droplet catcher assembly) as well as the orifices of the 
ink jet printing apparatus. Also, certain embodiments cooperate with a 
start-up, maintenance and/or storage station to facilitate wet print head 
storage and/or the supply of pressurized air for cleaning of the print 
head assembly.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 illustrates schematically an exemplary ink jet printing apparatus 1 
with which the present invention is useful. In general, the apparatus 1 
comprises a paper feed and return sector 2 from which sheets are 
transported into and out of operative relation on printing cylinder 3. The 
detail structure of paper feed and return components do not constitute an 
essential part of the present invention and need not be described further. 
Also illustrated generally in FIG. 1 is the apparatus print head assembly 
5 which is mounted for movement along carriage assembly 6 by appropriate 
drive means 7. During printing operation the print head assembly is 
traversed across a print path in closely spaced relation to a print 
substrate, e.g. a paper sheet, which is rotating on cylinder 2. Ink is 
supplied to and returned from the print head assembly by means of flexible 
conduits 11 which are coupled to ink cartridges 8. A storage, start-up 
and/or maintenance station 9 is constructed adjacent the left side (as 
viewed in FIG. 1) of the operative printing path of print head assembly 5. 
The drive means 7 and carriage assembly 6 are constructed to transport the 
print head assembly into operative relations with station 9 at appropriate 
sequences (e.g. storage, start-up or maintenance). 
Referring to FIG. 2, one embodiment of print head assembly 5 according to 
the present invention can be seen in more detail. The assembly 5 includes 
an upper print head portion including a print head body 21 mounted on 
housing 22 for movement by the carriage assembly 6. The body 21 has an ink 
inlet passage 23 leading to a print head cavity 24 and an outlet (not 
shown), leading from the cavity 24 to an ink recirculation system. The 
upper print head portion also includes an orifice plate 25 and suitable 
transducer means (not shown) for imparting mechanical vibration to the 
body 21. Such transducer can take various forms known in the art for 
producing periodic perturbations of the ink filament(s) issuing from the 
orifice plate 25, thus stimulating break-up of the ink filaments into 
streams of uniformly spaced ink droplets. One preferred construction for 
the print head body and transducer is disclosed in U.S. application Ser. 
No. 390,105, entitled "Fluid Jet Print Head" and filed June 21, 1982, now 
a continuation-in-part of Ser. No. 06/777,102, filed Sept. 17, 1985 in the 
name of Hilarion Braun; however, a variety of other constructions are 
useful in accord with the present invention. Preferred orifice plate 
constructions for use in accord with the present invention are disclosed 
in U.S. Pat. No. 4,184,925; however, a variety of other orifice 
constructions are useful. 
The lower portion of print head assembly 5 includes a charge plate 26, 
constructed to selectively impart charge to ink droplets at the point of 
filament break-up, and a drop catcher configuration 27 that is constructed 
and located to catch non-printing droplets (in this arrangement charged 
droplets). Exemplary preferred charge plate constructions are disclosed in 
U.S. application Ser. No. 517,608, entitled "Molded Charge Electrode 
Structure" and filed July 27, 1983, now abandoned, further filed as a 
continuation-in-part of Ser. No. 06/696,682, now U.S. Pat. No. 4,560,991 
in the name of W. L. Schutrum and in U.S. Pat. No. 4,223,321; however, 
other charge plate constructions are useful in accord with the present 
invention. Exemplary catcher configurations are described in U.S. Pat. 
Nos. 3,813,675; 4,035,811 and 4,268,836; again other constructions are 
useful. 
During the printing operation a plurality of ink filaments are ejected 
through the orifices in plate 25 and, under the influence of the 
transducer on body 21, break up into streams of uniformly sized and spaced 
droplets. The electrodes on charge plate 26 are addressed to selectively 
charge each droplet in each of the streams in accordance with information 
signals. In accord with the embodiment disclosed in FIG. 2, charged 
droplets are deflected onto the surface of catcher 27. The non-printing 
droplets which impact the catcher are recirculated back to the ink print 
head, while uncharged droplets pass on to the print substrate S as it 
rotates through the droplet impact zone Z of the apparatus. It will be 
appreciated that the print substrates (e.g. paper sheets) passing adjacent 
the print station cause a likelihood for dust (e.g. from, or carried by, 
the paper) coming into contact with the orifice plate, charge plate and 
catcher elements of the print head assembly. 
The FIG. 2 embodiment provides one construction, in accord with the present 
invention, for protecting those critical elements from such dust 
contamination. Thus, in the FIG. 2 embodiment wall means are provided for 
substantially enclosing the orifice plate 25, the charge plate 26, the 
catcher assembly 27 and a major portion of the ink droplet path from 
external air. In this embodiment the wall means, denoted in general 30, 
comprises a top wall portion 31, front and rear wall portions 32 and 33 
and side wall portions 34 (only one of which is illustrated in the FIG. 2 
cross-section). The wall means 30 is also constituted by surface 32a of 
the front wall 32 and the opposing surface 27a of catcher 27 so as to 
extend to a perimetrical region R that is closely adjacent the print path 
for substrate S. 
Also, according to the present invention, the FIG. 2 embodiment includes 
means defining an air inlet 39, which provides a passage for air flow into 
the region enclosed by wall means 30, and filtering means 40 for filtering 
air flowing through inlet 39 into the upper portion of the space 
substantially enclosed by wall means 30. In the FIG. 2 embodiment the 
filtering means 40 comprises air filters supported by upper sections of 
the wall portions 32 and 33; however the various other constructions that 
remove dust particles from air flowing into the region enclosed by wall 
means 30, can be utilized. 
The FIG. 2 embodiment of the invention is adapted to utilize predominantly 
the energy of the ink droplet streams to induce an air flow that protects 
the critical portions of its print head assembly from paper dust, etc. 
Specifically, the streams of ink droplets, which issue from the orifice 
plate in the normal course of printing operations, entrain air along their 
flight paths and thus induce a zone of decreased air pressure within the 
lower region enclosed by wall means 30. This low pressure zone in turn 
induces air external of the wall means 30 to flow through inlet passages 
39, and filters 40, along the paths indicated by arrows "A" in FIG. 2. 
Thus a continuous air stream exits at the perimetrical region R of housing 
30, and prevents dust particles associated with the print medium from 
moving inside the housing. The charge and orifice plates and the droplet 
catcher surfaces are therefore protectively air-screened using energy of 
normal printing functions. 
The air flow induced by approximately 60 droplet streams (comprised of 
droplets with 0.006" spacings and traveling at about 10 meters/sec.) has 
been found to work well in protecting the critical print head elements 
from debris with a droplet exit width (between 27a and 32a at region R) of 
about 0.03 inches. However, other droplet stream parameters will function 
effectively. In the FIG. 2 embodiment, it is highly preferred that the 
walls 27a and 32a are sufficiently closely spaced and cooperatively 
configured so that the flow of filtered air A passing therebetween is 
laminar in the direction of the droplet stream. 
FIG. 3 illustrates an embodiment of the invention wherein another normal 
printing function (viz relative movement between the print head and print 
medium) is employed to induce protective air flow for critical print head 
structures. More particularly, as the print substrate S is fed rapidly 
past the print zone by transport 3', it creates a film of boundary layer 
air traveling with it. By constructing lower surface 32b (which forms a 
downstream transverse portion of wall means 30) to be further spaced from 
the transport 3' than the lower surface 27b of catcher 27 (which forms an 
upstream transverse portion of wall means 30), an air control zone is 
defined for the boundary air film passing the print head assembly. Thus, a 
low pressure region is generated upstream of the catcher 27 by the 
entrainment of the air the boundary layer air flow and the constriction 
and expansion of this combined air flow. This low pressure region induces 
the siphoning of air through inlets 39 and along the path indicated by 
arrows "A". In an embodiment such as shown in FIG. 3, air flow induced by 
print substrate movement of about 80 in./sec. or more has been found 
sufficient to provide protection of the print head assembly independent of 
any ink jet stream operation. Lower velocities are useful to provide 
enhancement of the air flow with the ink streams operating. One useful 
spacing configuration of the wall means vis-a-vis the print substrate 3' 
(with substrate velocities in the range of about 80-120 inches per second) 
is for the upstream wall portion 27b to be about 0.025 inches from the 
substrate passing the print zone and for the downstream wall portion 32b 
to be about 0.060 inches from the substrate, with the spacing between 27b 
and 32b about 0.080 inches. Various other spacings that provide a 
constriction of the air moving with the substrate, followed by an 
expansion proximate the region where ink droplets leave the protection of 
wall means 30, will be useful in accord with the present invention. 
In the FIG. 3 embodiment of the present invention, the interior surface 
configuration of the lower portions of wall means 30 are constructed to 
increase in cross-sectional dimension from a relatively constricted air 
flow region proximate the charge plate 26 to a relatively expanded region 
at the perimetrical region adjacent the print path. This configuration is 
useful to provide high velocity air flow proximate the charge plate 26 
without causing disruptive turbulence within wall means 30. 
The FIG. 3 embodiment thus can rely predominantly on the energy of the 
transport medium to induce a filtered protective air flow for the critical 
elements of the print head assembly. This aspect is useful in applications 
where continuous jet streams are not always operating, e.g. in drop on 
demand jet printers or continuous printers which have periods wherein the 
print head is over moving print media with their flow inducing jet streams 
not operating. In some applications it may be desired to utilize both the 
energies of the ink jet stream(s) and the relative movement between the 
print head and the print substrate to maintain continuous flow of 
protective air. 
Considering now both the FIGS. 2 and 3 embodiments, it has been found that 
the wall means 32 provides another highly desired function. Thus, as the 
combined flow, of protective air from within wall means 30 and the air 
driven by the print medium, passes from beneath surface 32b, it enters 
another expansion region. This results in a vortex flow pattern downstream 
of wall 32 and that wall is important to shield the droplet flight path 
from the influence of the unstable vortex flow. 
FIG. 4 illustrates another embodiment of the present invention wherein the 
wall means 60, inlet passage 61 and filter 62 are adapted to provide 
protective air flow as described with respect to FIGS. 2 and 3 and 
additionally to cooperate with a storage and start-up 70 station of the 
printer apparatus. Elements which can be substantially the same as 
described with respect to FIGS. 2 and 3 are given the same numeral as 
previously used. 
The storage and start-up station 70 is shown in FIG. 4 and in general 
comprises a housing 71 having an ink sump cavity 72 and an air inlet 
passage 73 formed therein. A sealing member 74 is located around an upper 
portion of the housing in a configuration adapted to provide a peripheral 
seal around the ink stream outlet of the wall means 60 when the print head 
assembly is moved into engagement with station 70. A check valve 75 is 
located in air inlet 73 and biased to a normally closed condition. The 
upper portion of air inlet conduit 73 has a male portion 76 and a seal 77 
that are adapted to interfit with a start-up air inlet 66 in the housing 
60. The functions of station 70 are described in detail in concurrently 
filed U.S. application Ser. No. 06/722,551, entitled "Ink Jet Printing 
System Having a Wet Storage System" and concurrently filed U.S. 
application Ser. No. 06/722,545, entitled "Ink Jet Printing Apparatus 
Having an Improved Start-Up System", which are incorporated herein by 
reference. In general the station 70 provides for sealing the orifice and 
charge plates and catcher assembly from the external atmosphere during 
non-use and for introducing pressurized air through conduit 73 to skive 
clean the charge plate and catcher assembly during maintenance and 
start-up cycles. 
The embodiment of the present invention shown in FIG. 4 illustrates the 
advantages of the print head structure, such as described above, for 
cooperating in the storage and start-up functions, while maintaining the 
capabilities of siphoned protective air flow during normal printing 
operations. Thus, the enclosed upper chamber formed by walls means 60 of 
the FIG. 4 embodiment is divided into an upper and lower plenun "U" and 
"L" by an interior wall 67 having a passage 68 between the plenums. A 
spool valve 69 is mounted in passage 68 and is spring-biased to a downward 
position wherein air can flow between the upper and lower plenums through 
passage 68 (i.e. through spool valve 69) and the inlet 66 is closed by 
valve 69. As shown by arrows "A" in FIG. 4, induced air flow through 
filter 62 can therefore pass down into lowre plenum L and through the ink 
stream outlet as previously described. Thus protective air flow is 
provided during printing. 
During storage and start-up operations the valve 69 closes the 
communication between the upper and lower plenums (as shown in FIG. 4) so 
that the pressurized air from conduit 73 is directed past the orifice and 
charge plates and out through the ink stream outlet. Because upper plenum 
is closed the pressurized air from conduit 73 does not escape from outlet 
61. 
FIGS. 5 and 6 disclose another preferred embodiment of the present 
invention which is constructed to provide a common inlet for the 
protective air flow induced by the droplet streams and for the pressurized 
air supplied by storage and start-up station 80. As shown in FIG. 6, the 
common inlet 91 is formed in a portion of the wall means 90 that is 
extended downstream from the ink stream outlet and has a filter 92 as 
previously described. The outlet is adapted to cooperate with an air inlet 
conduit 83 of the station 80 to receive pressurized air and the walls 
around the ink stream outlet of the print head assembly are adapted to 
make in sealing relation with sealing means 84 of station 80. This 
embodiment is desirable for eliminating the need for separate plenums 
within the upper wall means and the valving interaction with the home 
station that was described with respect to FIG. 4. The downstream location 
of the inlet 91 positions the filter 92 so that paper dust does not 
readily clog it. This embodiment also lessens the flow restriction of air 
siphoned into the enclosure of wall means 90. 
The invention has been described in detail with particular reference to 
preferred embodiments thereof, but it will be understood that variations 
and modifications can be effected within the spirit and scope of the 
invention.