Filter arrangement for ink jet head

In the embodiments described in the specification, an ink jet head has an orifice filter disposed between a pressure chamber and an orifice to trap contaminant particles having a size likely to block the orifice, while permitting smaller particles to pass through the filter, thereby preventing blocking of the orifice while avoiding substantial pressure drop in the pulses producing ejection of drops from the orifice. In addition, a separate filter having a substantially smaller pore size is incorporated in a reservoir from which ink is supplied to the pressure chamber, thereby filtering out small particles which might accumulate to produce orifice-blocking particles.

BACKGROUND OF THE INVENTION 
This invention relates to arrangements for filtering ink in ink jet heads 
to prevent clogging of ink jet orifices. 
In ink jet systems, ink is ejected in the form of drops from a series of 
small orifices which may have a diameter of, for example, 0.020-0.060 mm, 
in response to selectively applied electrical pulses actuating a pressure 
transducer in a pressure chamber adjacent to each orifice. After ejection 
of an ink drop from an orifice, the ink in the associated pressure chamber 
is replenished from an ink reservoir in the ink jet head, which in turn is 
periodically refilled from a remote ink supply. 
One of the fundamental practical problems with ink jet technology is 
clogging of the small orifices in the ink jet head with contaminants 
carried in the ink flowing to the orifices. Such contaminants may 
originate in the ink supplied to the ink jet head or in air supplied to 
the ink reservoir as ink is withdrawn from it or they may be introduced 
into the ink passages in the ink jet head during assembly of the head. 
Regardless of the source of contaminants which can clog the orifices, it 
is essential to prevent such orifice-clogging, since the clogging of even 
a single orifice in an array containing, for example, 96 orifices, renders 
the head useless. 
To avoid orifice-clogging by contaminants in the ink supplied to an ink jet 
head, the ink reservoir in the head normally contains a fine filter having 
a pore size substantially smaller than the orifice size, for example, 
0.002-0.005 mm, by which solid particles of larger size entering the 
reservoir are screened out and prevented from reaching the orifices. 
Because such reservoir filters extend over a large area, and also because 
the reservoir is normally open to the atmosphere, the flow of ink from the 
reservoir to the pressure chambers and the orifices is not inhibited by 
the small size of the pores in the filter. Such reservoir filters, 
however, cannot prevent contaminant particles which are introduced into 
the ink jet head during assembly, or large agglomerations of small 
particles which can pass through the reservoir filter and accumulate, from 
becoming lodged in and blocking an orifice. 
One possible approach to solving the orifice-clogging problem is to place a 
filter at the inlet to the pumping chambers of the head where it will 
block orifice-size contaminant particles without interfering with the 
pressure pulse transmitted from the pumping chamber to the orifice to 
eject a drop from the orifice. This approach, however, has several 
drawbacks. First, since the pressure chamber is refilled with ink after 
drop ejection as a result of the relatively low negative pressure 
generated by the meniscus in the orifice, only a small pressure drop can 
be tolerated across a filter in the passage leading from the ink reservoir 
to the pressure chamber. Typically, such a filter must be designed to 
provide a pressure drop substantially less than half the meniscus 
pressure, for example, about 10% of the meniscus pressure. Second, a 
filter in the passage leading to the pressure chamber will inevitably trap 
air bubbles which cannot pass through the filter until a pressure greater 
than the bubble pressure is applied across the filter, which does not 
normally occur in the absence of purging. Third, a filter designed to 
produce a low pressure loss will generally have a low flow rate, which may 
make it susceptible to loading by poorly-dispersed ink components or 
contaminant particles which are smaller than the pore size of the filter. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide a new and 
improved filter arrangement for an ink jet head which overcomes the 
above-mentioned disadvantages of the prior art. 
Another object of the invention is to provide a filter arrangement for an 
ink jet head which substantially eliminates blocking of ink jet orifices 
by particles contained or formed within the head. 
These and other objects of the invention are attained by providing an ink 
jet head having an ink reservoir, a pumping chamber supplied with ink from 
the ink reservoir and an orifice plate with an orifice of selected 
diameter to produce an ink drop in response to pressure generated in the 
pressure chamber, and an orifice filter disposed between the pressure 
chamber and orifice in the orifice plate having a pore size smaller than 
the size of the orifice in the orifice plate. Preferably, to provide 
minimum pressure loss, the orifice filter has a pore size approaching the 
diameter of the orifice, for example, from 30%-100% of the orifice 
diameter and, more desirably, 75%-90% of the orifice diameter, to permit 
any contaminants significantly smaller than the orifice diameter to pass 
through the orifice filter while trapping contaminants approximating the 
size of the orifice. 
In this way, the limitation on filter resistance imposed by the 
meniscus-driven pumping chamber refill fluid dynamics of the ink jet head 
is avoided since the orifice filter is located downstream from the pumping 
chamber and its resistance may therefore be lumped with the higher pumping 
chamber and orifice resistances. Thus, if the resistance of the orifice 
filter disposed between the pumping chamber and the orifice is smaller 
than that of the pumping chamber and the orifice, it should have a 
negligible impact on the pumping chamber refill and therefore the jetting 
performance of the ink jet head. 
Furthermore, in addition to trapping orifice-size particles before they 
reach the orifice, a filter located immediately adjacent to the ink jet 
orifice has the further advantage that it provides a capillary barrier to 
depriming of the orifice because its high bubble pressure prevents loss of 
the meniscus at the orifice. Also, because it is between the pumping 
chamber and the orifice, the filter is subjected to liquid flow in 
opposite directions at relatively high rates during the fill-and-fire 
cycle of the pumping chamber, which should be effective to prevent loading 
of the filter with anything except the largest particles which could block 
the orifice.

DESCRIPTION OF PREFERRED EMBODIMENT 
In the typical embodiment of the invention schematically illustrated in 
FIG. 1, an ink jet head 10 has a series of orifices 11, only one of which 
is visible in the sectional view of FIG. 1, through which drops of ink are 
ejected in the usual manner in response to selective actuation of the 
portion of an electromechanical transducer 12 adjacent to a pressure 
chamber 13 containing ink. Ink is supplied to the pressure chamber 13 
through a conduit 14 leading from a deaerator 15 of the type described, 
for example, in the Hine et al. U.S. Pat. No. 4,940,995, which is 
incorporated by reference herein. As described in that patent, negative 
pressure is applied to vacuum plenums 16 on opposite sides of an ink path 
17 bounded by air-permeable, ink-impermeable membranes 18 in the deaerator 
to extract dissolved air from the ink, the vacuum plenums being connected 
through a line 19 to a pump unit 20 which is also arranged to apply 
positive air pressure through a line 21 for purging purposes. 
When the pressure chamber 13 is inactive, i.e., not being contracted to 
eject ink drops through the orifice 11, deaerated ink passes from the 
deaerator 15 through a conduit 22 to a flow-through passage 23 and an 
orifice passage 24 back to the pressure chamber 13 so as to circulate the 
deaerated ink back to the pressure chamber, thereby avoiding the formation 
of bubbles when negative pressure is applied by the transducer 12 to the 
ink in the chamber 13 during operation of the system. 
A reservoir 25 containing a supply of ink 26 is connected through an 
aperture 27 to the deaerator passage 17 so as to supply ink to the 
deaerator as the ink drops are ejected through the orifice 11 during 
operation of the system. When a low ink level in the reservoir 25 is 
detected by a low-ink detector 29, the ink in the reservoir 25 is 
replenished through a line 30 from a remote ink supply. 
The ink jet orifice 11 is designed to project an ink drop having a selected 
small volume which depends upon the type of system in which the ink jet 
head is utilized, and the orifice may have a diameter of, for example, 
about 0.020 mm to 0.060 mm. In order to prevent contaminants in ink 
supplied from the remote supply through the line 30 to the reservoir 25 
from being distributed in the ink in the reservoir and in the passages in 
the ink jet head, a reservoir filter 31 having an area substantially 
coextensive with the longitudinal cross-sectional area of the reservoir is 
positioned adjacent to one wall of the reservoir so as to intercept all of 
the ink introduced through the line 30. 
Conventionally, the filter 31 has a pore size substantially smaller than 
the diameter of the orifices 11 in the head, for example, 0.002 to 0.010 
mm, so as to trap all contaminants which might accumulate to form larger 
particles approximating the size of the orifices 11 and block the orifices 
or otherwise interfere with the operation of the system. Despite the 
presence of the filter 31, however, it is still possible for contaminant 
particles to block the orifices 11, either by way of introduction of 
particles through an air vent 32 through which air is drawn into the 
reservoir as the ink 26 is used, or because such particles become 
introduced and are trapped in the ink passages such as the ducts 14, 17, 
22, 23 and 24 in the ink jet head during assembly of the head. Although 
the observance of extensive clean-room procedures during the manufacture 
of the ink jet head can reduce the possibility of such contaminant 
particles being incorporated into the head, it cannot completely eliminate 
that possibility. 
Consequently, in accordance with the invention, an orifice filter 33 is 
incorporated into the ink jet head in the orifice passage 24 between an 
orifice plate 34 containing the orifices 11 and the adjacent portion of 
the ink jet head 35 containing the passages through which ink flows to the 
orifices during operation. The orifice filter 33 has a pore size selected 
to trap only those particles which might clog the orifices 11 while 
permitting smaller particles to pass through, thereby avoiding substantial 
pressure losses. Thus, the size of the pores in the orifice filter 33 must 
be smaller than the orifice size but larger than the pores in the 
reservoir filter 26 and is preferably about 30%-100% and, desirably, about 
75%-90% of the diameter of the orifice 11. Accordingly, if the ink being 
ejected through the orifice contains particles smaller than the pore size, 
they will not be blocked by and will not tend to clog the orifice filter 
33, but instead will be ejected with the ink drop passing out of the 
orifice 11, thereby reducing the tendency of the filter 33 to become 
clogged and produce an undesirable pressure drop. 
In the preferred embodiment, the filter material is selected for 
compatibility with the ink and printhead fabrication processes. For 
example, the material may be etched or electroformed out of a metal such 
as nickel, gold or copper. Alternatively, the filter may be laser-machined 
from a polymer film such as polyimide or Teflon. 
With this arrangement, the pressure drop is thus minimized and the presence 
of the orifice filter 33 will reduce the velocity of a drop ejected 
through the orifice by only about 5%-10% which, if necessary, can be 
compensated by appropriate adjustment of the actuation of the transducer 
12. 
FIG. 2 illustrates an arrangement of an orifice filter in the orifice 
passage in greater detail. In this case, the ink passages in the head are 
formed by a series of adjacent plates having appropriate openings or 
channels which are sandwiched together. Thus, for example, the pressure 
chamber 13 is formed in a cavity plate 36 disposed adjacent to a stiffener 
plate 37 and the flow-through passage 23 is formed in a flow-through plate 
38. In addition, the orifice filter 33 is mounted between a membrane plate 
39 and a shim 40 to which the orifice plate 34 is affixed. The shim 40 
may, for example, be made of pyrolytic carbon to provide high thermal 
conductivity which facilitates the transfer of heat to hot melt ink in the 
orifice passage 24 to as to maintain it at the desired temperature for 
drop ejection. 
If the meniscus 41 in the orifice 11 is drawn back into the orifice passage 
24 during refill of the pressure chamber 13 or for any other reason, the 
filter 33 is also effective to prevent the meniscus from forming a bubble 
which could move through the passage 24 into the pressure chamber 13 and 
interfere with the proper operation of the system. Moreover, because the 
ink in the passage 24 flows first in one direction during ejection of a 
drop and then in the opposite direction during refilling of the pressure 
chamber 13 at relatively high flow velocities, accumulation and loading of 
the filter 33 with smaller particles which would not tend to block the 
orifice 11 is effectively prevented. 
Although the invention has been described herein with reference to a 
specific embodiment, many modifications and variations therein will 
readily occur to those skilled in the art. Accordingly, all such 
variations and modifications are included within the intended scope of the 
invention.