Ink Jet Writing Head With Spacer in Capillary Chamber

The present invention relates to apparatus for applying ink droplets to a writing surface, and more particularly to an ink jet writing head which provides consistent writing performance by accommodating machining errors.

U.S. Pat. No. 4,106,032, granted to Masayoshi Miura et al. and assigned to the same assignee as the present invention, discloses an ink jet writing head comprising a liquid chamber divided into an inner chamber portion and a disc-like outer chamber portion by a dividing plate having a connecting channel which communicates the inner and outer chamber portions. The inner chamber portion is defined by a piezoelectric transducer to create pressure rises therein and the outer chamber portion is open to the atmosphere through an ink discharge channel that is adjacent and axially aligned with the connecting channel so that upon application of an electrical pulse to the transducer a pressure rise occurs in the inner chamber portion which propagates toward the ink discharge channel producing a stream of ink droplets expelled from the discharge channel toward a writing surface. The outer liquid chamber portion is connected to an ink supply source so that upon the termination of the electrical pulse the ink in the outer chamber portion is sucked in toward the discharge channel. It is known that this pressure rise encounters losses at various points as it propagates toward the discharge channel and that the total amount of such losses accounts for the minimum threshold voltage of the writing head. The thickness of the disc-like outer chamber portion usually ranges from 50 to 400 micrometers to provide capillary action. Although the diameters of the connecting channel and the discharge channel can be precisely machined, the thickness of the disc chamber portion tends to vary in an appreciable range of values due to difficulties encountered in assemblage of writing heads. The inconsistent values of thickness results in different writing heads having different propagation losses. It is found that a deviation of a 1 micrometer in the thickness value results in a peak-to-peak voltage variation of about 1 volt in the threshold level. Such thickness variations also affect the amount of ink discharged per unit time. Particularly, when the disc-like outer chamber portion has different thicknesses across its transverse dimension, the amount of discharged liquid varies from time to time.

The disclosed ink jet writing head further includes an air chambaer which is open to the atmosphere through an air discharge channel that is aligned with to the ink discharge channel to provide a laminar air flow for directing the discharged ink droplets toward the writing head. To provide this laminar air flow, the air chamber is connected to a source of pressurized air which is also connected to the source of ink. The pressure in the air chamber is adjusted so that it is statically balanced against the liquid pressure inside the ink discharge channel. This serves to lower the surface tension of the ink in the discharge channel and hence the minimum threshold voltage. The lowering of minimum threshold voltage is advantageous in that it significantly improves the gradation of reproduced image. This air chamber is also formed into a disc-like chamber, the thickness of which is also susceptible to manufacturing errors.

It is also known that the threshold voltage of a given writing head is variable as a function of the thickness value of the disc-like air chamber, so that variation in the latter value results in different writing heads having different values of threshold voltage. Therefore, the prior art ink jet writing head suffers from inconsistent operating performances which required cumbersome adjustments when writing heads are interchanged.

Accordingly, the primary object of this invention is to provide an ink jet writing head assuring consistent writing performance regardless of errors which might be produced during machining or other production processes of the writing head.

The ink jet writing head of the present invention comprises a housing which is shaped to form an inner liquid chamber for containing ink to be applied to a surface and having an electromechanical transducer for generating short-duration ink pressure rises in the inner liquid chamber, a dividing plate having therein a connecting channel and secured to the housing opposite to the transducer, a nozzle plate having therein an ink discharge nozzle axially aligned with the connecting channel and secured to a surface of the housing axially spaced from the dividing plate to define an outer liquid chamber. The axial dimension of the outer liquid chamber provides capillary action for ink supplied from an ink source. According to the invention, the writing head further includes spacer means provided in the outer liquid chamber to maintain the axial dimension of the outer liquid chamber at a predetermined value regardless of errors produced during manufacture of the housing and assemblage of the writing head.

The spacer means preferably includes a plurality of angularly spaced and radially extending segments. From the manufacturing standpoint, the spacer means preferably comprises a single plate member having the thickness of the outer liquid chamber and a cutout portion shaped to form a plurality of angularly spaced apart radially extending spacer segments. The spacer segments are located between the ink nozzle plate and the dividing plate. The outer periphery of the plate member is adjacent or in contact with the boundary of the outer liquid chamber so that the plate member is secured in position during manufacture.

The ink jet writing head of this invention may further include an air nozzle plate having therein an air nozzle axially aligned with the ink nozzle. The air nozzle plate is secured to the housing to define an air chamber in communication with a source of pressurized air. A second spacer means is provided in the air chamber. This spacer means may be of identical configuration to but of different size from the spacer means provided in the outer liquid chamber.

The invention will be further described by way of example with reference to the accompanying drawings, in which:

FIG. 1 is an illustration of the ink jet writing apparatus utilizing an ink jet writing head embodying the invention shown in axial cross-section;

FIGS. 2A to 2C are illustrations of spacers in cross-section taken along the lines 2--2 of FIG. 1;

FIGS. 3A to 3C are illustrations of spacers in cross-section taken along the lines 3--3 of FIG. 1;

FIG. 4 is an illustration of a partial cross-sectional view of the ink jet writing head of FIG. 1 in a modified form;

FIG. 5 is an illustration of the ink nozzle plate in cross-section taken along the lines 5--5 of FIG. 4;

FIG. 6 is an illustration of the air nozzle plate in cross-section taken along the lines 6--6 of FIG. 4;

FIG. 7 is an illustration of a partial cross-sectional view of the ink jet writing head of FIG. 1 in a further modified form;

FIG. 8 is an illustration of the dividing plate in cross-section taken along the lines 8--8 of FIG. 7; and

FIG. 9 is an illustration of the ink nozzle plate in cross-section taken along the lines 9--9 of FIG. 7.

In FIG. 1 the ink jet writing apparatus embodying the present invention comprises a writing head 10, a liquid or ink supply source 11 and a source of pressurized air 12. The ink jet writing head 10 comprises an outer liquid chamber 13, an inner liquid chamber 14 and an air chamber 15. The outer liquid chamber 13 comprises a disc-like capillary chamber portion 13a and an annular chamber portion 13b encircling the capillary chamber portion 13a. The annular liquid chamber portion 13b is connected to ink supply source 11 with a liquid inlet conduit 16. Similarly, the air chamber 15 comprises a disc-like laminar flow air chamber portion 15a and an annular air chamber portion 15b encircling the laminar flow chamber portion 15a. The annular air chamber portion 15b communicates with the pressurized air source 12 via air inlet conduit 17. The inner and outer liquid chambers 14 and 13 are interconnected by a connecting channel 18 provided in a dividing plate 19. The air chamber 15 is open to the atmosphere via an air nozzle 22 formed in a nozzle plate 23 to provide a laminar air flow toward a writing surface located forward of the writing head. The outer liquid chamber 13 is open to the atmosphere via an ink nozzle 20 formed in a nozzle plate 21 and through the air chamber 15 and air nozzle 22. The ink nozzle 20 axially aligned with and adjacent both connecting channel 18 and air nozzle 22.

The inner liquid chamber 14 has a rearwardly increasing diameter portion in contact with a piezoelectric transducer formed by a piezoelectric member 24a and an electrode 24b connected to a signal source 25.

The application of an electrical pulse to transducer 24 generates a short-duration pressure rise within inner chamber 14 to cause ink in outer chamber 13 to be ejected outwardly from writing head 10 through aligned nozzles 20 and 22 in a stream of droplets confined within a passage formed by the laminar air flow. The pressurized air is also applied through a conduit 26 to the ink supply source 11 to increase the pressure inside the outer liquid chamber 13. This pressure is regulated by a regulating valve 27 provided in the conduit 26 so that it is statically balanced against the pressure in the portion of the air chamber 15 adjacent ink nozzle 20. This feature serves to lower the surface tension of the liquid boundary layer in the ink nozzle 20 to facilitate the ejection of ink, thereby lowering the minimum threshold voltage of the writing head.

In accordance with the present invention, first spacer means 40a, as shown in FIG. 2A, is located in the disc-like liquid chamber portion 13a to determine the thickness or axial dimension of portion 13a. In this embodiment, spacer means 40a includes a plurality of elongated spacer segments which are angularly spaced apart with respect to connecting channel 18. Each spacer segment 40a extends in radial direction and is formed with a tapered portion pointed toward connecting channel 18 to allow the ink in the annular chamber portion 13b to flow through the disc-like chamber portion 13a to the connecting channel 18.

In a practical aspect of the writing head, the dividing plate 19 is cemented to an annular surface 28 of the writing head housing 10a. Surface 28 is machined so that it is spaced rearwardly a predetermined distance from an annular surface 29 to which the ink nozzle plate 21 is cemented. This fabrication involves the use of a suitable adhesive material applied to the contacting surfaces. The spacer means 40a is cemented to the dividing plate 19 prior to securing nozzle plate 21 to surface 29. With spacer means 40a, machining errors in the distance between surfaces 28 and 29 can be accommodated by appropriately adjusting the thickness of the adhesive layer on the contacting surface 29. Thus, the thickness of disc-like chamber portion 13a is maintained to a degree of precision not attainable with the prior art.

In the same manner as descried above, the disc-like air chamber portion 15a is provided with spacer means 50a, as shown in FIG. 3A. The spacer means 50a also includes a plurality of angularly spaced apart, radially extending segments each having a tapered portion pointed toward ink nozzle 20. Air flow passage is thus not obstructed by the spacer segments 50a. Segments 50a are cemented by adhesive to the nozzle plate 21 after the latter is secured in place in the manner just described. With the spacer means 50a being secured in position, the air nozzle plate 23 is cemented to an annular surface 30 by adhesive material in the form of a layer having an appropriate thickness to accommodate machining errors which might exist in the distance between the annular surfaces 29 and 30, so that the thickness of the disc-like chamber 15a is maintained to a degree of precision not attainable with the prior art.

FIGS. 2B, 2C, 3B and 3C are illustrations of modified forms of spacer means of FIGS. 2A and 3A, wherein the same reference numerals with different alphabetic subscripts are used to indicate corresponding parts. In FIG. 2B, spacer means 40b has a generally disc-like configuration having an inner cutout portion 41 which radially extends beyond the periphery of the dividing plate 19 to allow the ink in the annular chamber 13b to flow into the capillary chamber portion 13a. The outer periphery of the spacer disc 40b is adjacent to or in contact with the outside wall of the annular liquid chamber portion 13b so that the disc 40b is secured in position during manufacture. The cutout portion 41 has a pair of straight edges 43 which define cross-hatched areas 42 with the outer periphery of the dividing state 19. The portions of the rear side of spacer 40b which are cross-hatched are in contact with or cemented to the dividing plate 19, while the entire surface of the front side thereof is in contact with or cemented to the ink nozzle plate 21.

In FIG. 3B, spacer means 50b is of a disc-like configuration having a cutout portion 51 which radially extends beyond the outer periphery of the disc-shaped ink nozzle plate to allow air to introduce thereinto from the annular air chamber portion 15b. Straight edges 53 of the cutout portion 51 defines cross-hatched areas 52 with the outer periphery of the disc-shaped ink nozzle plate 21. The portions of the rear side of spacer 50b which are cross-hatched are in contact with or cemented to the ink nozzle plate 21, while the entire surface of its front side is in contact with or cemented to the air nozzle plate 23. The outer periphery of the spacer 50b is in contact with or adjacent to the outside wall of the annular air chamber portion 15b so that the spacer 50b is secured in position during manufacture.

Spacer means 40c and 50c of similar construction shown respectively in FIGS. 2C and 3C are a preferred form of the invention. In FIG. 2C, spacer 40c comprises an annular member having a plurality of integrally formed spacer segments which radially inwardly extend beyond the periphery of the dividing plate 19. These inwardly extending portions are in contact with or cemented to the dividing plate 19 to serve as spacer elements. The outer perimeter of the ring-shaped portion is adjacent or in contact with the outside wall of the annular liquid chamber portion 13b to secure the spacer 40c in place during manufacture, and the inner perimeter of the ring-shaped portion lies within the area of the annular ink chamber 13b to secure communication between the latter and the capillary chamber 13a. The inwardly extending segments are preferably equi-angularly spaced apart with respect to the ink nozzle 18 so that the rate of ink flow toward the nozzle 18 is substantially equal in all directions.

In FIG. 3c, the outer perimeter of annular spacer 50c is adjacent to or in contact with the outside wall of the annular air chamber portion 15b, and the inner perimeter thereof lies within the annular chamber portion 15b to secure communication between the latter and the air chamber portion 15a. The plurality of spacer segments extend inwardly between the periphery of the ink nozzle plate 21 so that the inwardly extending portions serve as spacer elements between the nozzle plates 21 and 23.

FIGS. 4 to 6 are illustrations of further embodiments of the present invention wherein those corresponding to the elements of FIG. 1 are indicated by corresponding numerals. In FIG. 4, in which a partial cross-sectional view of writing head 10 is shown, spacer elements are integrally formed with the ink and air nozzle plates 21a and 23a respectively. As clearly shown in FIG. 5, the ink nozzle plate 21a is formed on the rear side thereof with a recess 60 with a depth equal to the thickness of the capillary action chamber 13a. The recess 60 includes angularly spaced apart portion which extend radially outwardly from the ink nozzle 20 forming the capillary chamber 13a and further extend beyond the periphery of the dividing plate 19 but not beyond the periphery of the nozzle plate 21a. Thus, the outwardly further extending portions of the recess 60 communicate with annular liquid chamber portion 13b. Therefore, the nozzle plate 21a is in contact with the dividing plate 19 in areas as cross-hatched in FIG. 5 to serve as the spacer elements.

In FIG. 6, the air nozzle plate 23a is formed on the rear side thereof with a recess 61 having a depth equal to the thickness of the laminar flow air chamber 15a. The recess 61 includes angularly spaced apart portions which extend radially outwardly from the air nozzle 22 forming the laminar air flow chamber 15a and further extend beyond the periphery of the ink nozzle plate 21a but not beyond the periphery of the air nozzle plate 23a. Thus, the outwardly further extending portions of the recess 61 communicate with annular air chamber portion 15b. The nozzle plate 23a is thus in contact with the ink nozzle plate 21a in areas cross-hatched in FIG. 6 to serve as the spacer elements.

Because of the integral construction of spacer elements with the nozzle plates just described, the embodiments of FIGS. 4 to 6 simplify the assemblage of the writing head.

Alternative embodiments are shown in FIGS. 7 to 9. In FIGS. 7 and 8, the spacer elements indicated by the cross-hatched areas in FIG. 5 are integrally formed with the dividing plate 19a. More specifically, the dividing plate 19a is formed on its front side with a recess 62 having a depth equal to the thickness of the capillary chamber portion 13a. The recess 62 includes angularly spaced apart portions extending radially outwardly from the connecting channel 18 beyond the periphery of the dividing plate to secure communication between the annular liquid chamber portion 13b and the capillary chamber portion 13a which is formed by the recess 62. Similarly, as shown in FIGS. 7 and 9, the ink nozzle plate 21b is formed on its front side with a recess 63 having a depth equal to the thickness of the laminar air flow chamber 15a. The recess includes angularly spaced apart portions extending radially outwardly from the ink nozzle 20 forming the laminar air flow chamber 15a and further extends beyond the periphery of the ink nozzle plate 21b to secure communication with the annular air chamber portion 15b and the laminar air flow chamber 15a. The hatched areas in FIG. 9 are in contact with the air nozzle plate 23 to serve as the spacer elements.