Ink-jet printer with ink path and method of forming the ink path

An ink path through which ink is delivered from an ink source to a printhead unit includes an ink tube and a joint. The ink tube has a first layer formed of a material with low vapor and gas permeability and a second layer radially thicker than the first layer and formed of a flexible material. The joint has a maximum-diameter portion whose outer diameter is larger than an inner diameter of the ink tube. The joint is inserted into the ink tube. Further, a locking member is fitted over the ink tube. The locking member has an inner-diameter portion whose inner diameter is smaller than an outer diameter of a connection between the maximum-diameter portion of the joint and the ink tube.

BACKGROUND OF THE INVENTION

1. Field of Invention

The invention relates to an ink-jet printer and, more particularly, to an ink-jet printer having an ink path formed by ink path forming members that are securely, hermetically interconnected. The invention also relates to a method of forming such an ink path.

2. Description of Related Art

Ink-jet printers that incorporate an ink supply system using a tube are known. Such ink-jet printers have a printhead that ejects ink onto a printing medium, a carriage on which the printhead is mounted, an ink tank that is installed external to the carriage to store ink, and a tube through which ink is supplied from the ink tank to the printhead. One end of the tube is connected to the stationary ink tank, while the other end of the tube is connected to the printhead that reciprocates together with the carriage along a printing medium. Typically, the tube is connected to the printhead frictionally by inserting a joint member of the printhead into the tube.

However, a problem arises in the connection between the tube and the joint member when the printhead repeatedly reciprocates. If the tube moves randomly as the printhead reciprocates, the tube may be loosened or detached from the joint member to permit the entry of air into the ink path. Accumulation of air bubbles in the ink path may lead to a clogging of the ink path and an ink ejection failure.

The tube used for the above-described ink supplying system is typically formed of materials with low vapor and gas permeability, such as polyethylene (PE) and polypropylene (PP), to prevent evaporation of moisture contained in the ink and air permeation through the tube. Compared to tubes formed of flexible materials, such as ethylene rubber and butadiene rubber, the tube formed of the above-described materials, which are generally hard, makes poor contact with the joint member thereby permitting entry of air to the ink path through a gap between the tube and the joint member. As a result, the accumulation of air bubbles in the ink path may lead to a clogging of the ink path and an ink ejection failure.

Japanese Patent No. 2563784 is directed to ink path forming members in an ink-jet printer and discloses an air-tight connection between an ink supply tube and a pipe joint of an ink source or an ink receiver. The tube is inserted into an inner recess of the pipe joint, and the interconnected tube and pipe joint are securely locked by a locking member while a sealing member is interposed between the pipe joint and the locking member. Although the disclosed connecting structure provides an air-tight, secure connection between the ink path forming members, it is fairly complex and requires a large number of members.

SUMMARY OF THE INVENTION

The invention addresses the forgoing problems and provides an ink-jet printer having an ink path formed by ink path forming members that are simple in structure yet securely, hermetically interconnected.

One aspect of the invention provides an ink-jet printer that includes a printhead unit ejecting ink onto a printing medium, an ink source external to the printhead unit, and an ink path thorough which ink is delivered from the ink source to the printhead unit. The ink path includes first and second ink path forming members. The first ink path forming member has a head with a maximum-diameter portion and an open end tapered down from the maximum-diameter portion, and a neck extending from the head and having a smaller diameter than the maximum-diameter portion. The second ink path forming member is formed of at least a flexible elastic material and has an inner diameter smaller than the maximum diameter of the first ink path forming member. The head and the neck of the first ink path forming member are inserted into the second ink path forming member, and the second ink path forming member radially expands at the maximum-diameter portion and contracts at the neck of the first ink path forming member.

The second ink path forming member is a double-layer ink tube having a first layer formed of a material with low vapor and gas permeability and a second layer radially thicker than the first layer and formed of the flexible elastic material.

In another aspect of the invention, the ink path further includes a locking member fitted over the second ink forming member and having a first inner-diameter portion whose inner diameter is smaller than an outer diameter of a connection between the maximum-diameter portion of the first ink path forming member and the second ink path forming member. The first inner-diameter portion presses an outer periphery of the second ink path forming member and locks the connection.

Another aspect of the invention provides a method of forming the ink path through which ink is delivered from the ink source to the printhead unit. A filling liquid is first applied to either an outer periphery of the open end of a first ink path forming member or an inner periphery of the second ink path forming member. Then, the first and second ink path forming members are connected to each other by inserting the first ink path forming member into the second ink path forming member while keeping the filling liquid held between the outer periphery of the open end of the first ink path forming member and the inner periphery of the second ink path forming member.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

One embodiment of the invention will be described with reference to the accompanying drawings.

FIG. 1is a plan view showing the internal structure of an ink-jet printer1according to one embodiment of the invention. The ink-jet printer1includes, in its main frame2, a printhead unit3that ejects ink onto a sheet of paper, an ink tank4that stores ink to be supplied to the printhead unit3, tubes5through which ink is supplied from the ink tank4to the printhead unit3, a recovery unit6, and a sheet feeder that feeds sheets of paper.

The main frame2is substantially box-shaped and formed of flame-retardant plastic. A guide rod7is horizontally disposed in the longitudinal direction of the main frame2and supports the printhead unit3such that the printhead unit3reciprocates in direction A (right and left direction inFIG. 1) perpendicular to the sheet feed direction B.

The printhead unit3is substantially box-shaped, and includes a carriage3aand a housing3bcontinuously formed from the carriage3a. The housing3bhouses printheads (not shown), an air trap unit11(FIG.2), and other units.

The carriage3ais fitted onto the guide rod7so as to reciprocate thereon. A belt, attached to the carriage3a, is looped over rollers. When a carriage motor, which is connected to one of the rollers, rotates, the belt is driven to move the printhead unit3.

Feed rollers are provided below the printhead unit3to feed a sheet of paper. The feed rollers disposed at the front and rear of the printhead unit3feed a sheet of paper in a substantially horizontal direction indicated by arrow B when a feed motor rotates.

A plurality of printheads, for example, four printheads are provided side by side in the printhead unit3to perform full-color printing, with their ink nozzles facing down and open toward the sheet side. The printheads receive ink from the air trap unit11, which will be described later, and distribute ink to ink chambers provided for corresponding ink nozzles. Then, ink is ejected through the ink nozzles by the action of actuators, such as piezoelectric elements. The printheads are supported by the lower surface of the housing3b.

The ink tank4, disposed below the sheet feed path, stores ink to be supplied to the printhead unit3. The ink tank4consists of four ink tanks4a-4dthat hermetically contain black, yellow, cyan, and magenta inks, respectively. The ink tanks4a-4dare connected to the corresponding printheads through the corresponding tubes5a-5d.

The recovery unit6, disposed on the left side of the main frame2, performs a recovery operation for the printheads to restore the printheads to a normal ejection state. The recovery unit6includes a suction cap6a, a suction pump (not shown) that sucks ink from the printhead unit3through the suction cap6a, and a wiper6bthat wipes the ink nozzle surface of the printhead unit3.

The suction cap6ais substantially box-shaped and makes contact with and hermetically covers the ink nozzle surface. A discharge tube6cis connected to the bottom of the suction cap6a. Ink is sucked from the suction cap6aby the action of the suction pump, and flows out through the discharge tube6c. When the suction is completed, the suction cap6amoves away from the ink nozzle surface, and the wiper6b, formed by a rubber plate, wipes the ink nozzle surface smeared with ink. With that, the recovery treatment is completed.

Referring now toFIG. 2, the internal structure of the printhead unit3will be described.FIG. 2is a plan view of the printhead unit3. The printhead unit3contains the air trap unit11and a joint unit12.

The air trap unit11traps air bubbles generated in the tubes5. The air trap unit11is shaped like a rectangular solid and disposed in the middle of the housing3bof the printhead unit3. The air trap unit11is divided into four separate air traps30-33that correspond to the four printheads disposed below the air traps30-33. At the rear sides (top side inFIG. 2) of the air traps30-33, four joints34are provided substantially in a row so as to be connected to the joint unit12. The joints34are tapered down toward the joint unit12and each joint34has, in its inside, an ink inlet11ffor a corresponding one of the air traps30-33.

The joint unit12is provided to connect tubes5a-5dto the corresponding air traps30-33. The joint unit12is shaped like a rectangular solid and disposed behind the air trap unit11(above the air trap unit11in FIG.2). The joint unit12has four separate ink paths12a-12d. At both ends of each ink path12a-12d, a first joint35and a second joint36are provided in a protruding manner so as to be connected to the corresponding air trap30-33and tube5a-5d.

The first joints35are arranged substantially in a row on a surface of the joint unit12and face the joints34of the air traps30-33. Each first joint35has a neck projecting from the body of the joint unit12and a head radially extending from the neck and tapered down toward the corresponding joint34. Each first joint35and the corresponding joint34are inserted into a connecting member37from its opposite ends, and thereby connected to each other. The first joints35and the joints34are formed of a relatively inflexible material, such as polypropylene or other hard plastics. Connections between the joints34and the first joints35will be described later in detail with reference toFIGS. 3 and 4.

The second joints36are provided for the joint unit12, two for each of the right and left sides of the joint unit12. Each second joint36has a neck projecting from the body of the joint unit12and a head radially extending from the neck and tapered down toward the corresponding tube5a-5d. Each second joint36is inserted into one end of the corresponding tube5a-5d, and thereby connected to the corresponding tube5a-5d. The second joints36are formed of a relatively inflexible material, such as polypropylene or other hard plastics. Connections between the second joints36and the tubes5a-5dwill be described later in detail with reference toFIGS. 7-14.

FIGS. 3-4, and8-10show various structures designed to prevent troubles caused by air bubbles in the ink path formed between the tube5and the air trap unit11. If any gap, created by two ink forming members (joints), is not filled with ink and an air bubble remains there, very small bubbles dissolved in the ink will gather around the remaining air bubble to grow into a large air bubble. The large air bubble can possibly narrow or clog the narrow ink path and cause a poor ink supply and/or an ink ejection failure. The structures to be described are designed to prevent such failures.

Referring first toFIG. 3, a connection between the joint34of the air trap unit11and the first joint35of the joint unit12will be described.FIG. 3is an enlarged cross-sectional view showing a connection between the joint34and the first joint35. InFIG. 3, the first joint35is shown below the joint34and ink flows from the first joint35to the joint34in the directions of the arrows.

As described above, the joint34and the first joint35are connected by a connector37. The connector37is an elastic body in the form of a hollow cylinder. A ring-shaped sealing portion38projects from a middle part of the inner periphery of the cylinder. An ink path formed inside the joint34has an inner diameter d1of about 1.5 mm, the sealing portion38has an inner diameter d2of about 2.0 mm, and an ink path formed inside the first joint35has an inner diameter d3of about 2.2 mm.

The joint34is inserted from one end of the connector37to the sealing portion38, and the first joint35is inserted from the other end of the connector37to the sealing portion38. The joint34and the first joint35are surrounded by the connector37, and thereby connected to each other. The connector37radially expands at the head of the first joint35and contracts at the neck of the first joint35. At this time, the joint34and the first joint35are opposed to each other at their open ends, and the sealing portion38is sandwiched between the end face34aformed around an opening of the joint34and the end face35bformed around an opening of the first joint35.

When the joint34, the sealing portion38, and the first joint35are connected, their inner peripheries define an ink path. The ink path has no valley-like gaps between the end faces34a,35band becomes gradually narrower, from the first joint35to the joint34, in the direction of flow of ink. Accordingly, due to the different ink path diameters, steps39are formed facing the flow of ink at the connection between the joints34,35. Because ink flows toward the steps39, the velocity of flow of ink is kept fairly high, thus preventing accumulation of air bubbles at the steps39.

Referring now toFIG. 4, an alternate form of the joint34and the first joint35will be described.FIG. 4is an enlarged cross-sectional view of a joint134of the air trap unit11and a first joint135of the joint unit12when they are connected. The same elements designated and described inFIG. 3will not be described again.

In this alternate form, the joint134defines an ink path about 2.2 mm in inner diameter d1. The first joint135has a substantially cylindrical outer periphery and has a tapered recess135afacing the joint134to receive the joint134. The first joint135defines an ink path about 2.2 mm in inner diameter d3. A connector40formed by an elastic body is tapered at its inner and outer peripheries and has a sealing portion40aat its one end, which defines an ink path about 2.2 mm in inner diameter d2.

The connector40is brought into intimate contact with the tapered outer periphery of the joint134and with the tapered recess135aof the first joint135. The joint134is fitted into the recess135aof the first joint135and connected to the first joint135via the connector40. At this time, the sealing portion40ais sandwiched by the end face134aof the joint134and the inner end face of the recess135b. When the joint134, the connector40, and the first connector135are connected, their inner peripheries become flush with each other and define an ink path about 2.2 mm in inner diameter. Accordingly, no step or gap is formed between the end face134aand the inner end face135b, and thus an ink ejection failure due to accumulation of air bubbles is prevented. In addition, the connector40, formed by a resilient body, closely contacts the outer periphery of the joint134and provides a good seal around the joint134against the entry of air from the outside.

InFIG. 3, the joint34, the first joint35, and the sealing portion38may be designed to define an ink path that has a uniform inner diameter as in FIG.4. InFIG. 4, the joint134, the first joint135, and the sealing portion40amay be designed to define an ink path that has different inner diameters and becomes narrower in the direction of flow of ink in FIG.3. InFIGS. 3 and 4, it is preferable that the sealing portions38,40aare compressed between the end faces of the associated joints.

Referring now toFIGS. 5 and 6, the structure of the tube5used in the ink-jet printer1will be described.FIG. 5is an enlarged cross-sectional view of the tube5. The ink tube5is double-layered and has an inner layer50that contacts ink and an outer layer51fitted over the outer periphery of the inner layer50. The tube5preferably has an inner diameter D1of about 1.4 mm and an outer diameter D2of about 3.0 mm. In the ink-jet printer1, however, the tube5may have an inner diameter D1of between about 0.8-2.0 mm, and an outer diameter of between about 2.4-4.0 mm. The inner layer50may have a thickness D3of between about 60-80 μm, and preferably about 75 μm. The outer layer51is preferably more than twice as thick as the inner layer50to make the tube5kink resistant.

The inner layer may be formed of resins with low vapor and gas permeability, such as olefin base resins or fluorine base resins, namely, fluorinated ethylene propylene (FEP), polytetrafluoroethylene (PTFE), polyethylene (PE), and polypropylene. The inner layer is preferably formed of fluorinated ethylene propylene (FEP).

The outer layer may be formed of highly flexible and elastic olefin base rubber, silicon base rubber, or fluorine base rubber, such as silicon rubber and fluororubber (FKM). The outer layer is preferably formed of silicon rubber. The outer layer may have a Shore A hardness of about 60-80, and preferably about 70.

The following table shows the results of comparative tests conducted on single-layer and double-layer tubes formed of polyethylene (PE) and other materials.

In the experiments, single-layer tubes and double-layer tubes were set to have the same inner diameter D1and the same outer diameter D2. To evaluate ink drying properties, that is, vapor and gas permeability, tubes formed of various materials were filled with ink and left alone for about three months, and changes in ink weight were measured. In the table, • indicates the cases where changes in ink weight were very little, o indicates the cases where changes in ink weight were little, and Δ indicates the cases where changes in ink weight were noticeable. Additionally, to evaluate flexibility and buckling resistance, the tubes were bent repeatedly and checked for any tear or breakage. In the table, o indicates the cases where no tear or breakage was produced, and x indicates the cases where a tear or breakage was found.

Single-layer tubes formed of FEP, PTFE, and PE provided excellent results in the ink drying test, but provided poor results in the flexibility and buckling resistance tests. Single-layer tubes formed of FKM and silicon rubber provided poor results in the ink drying test, but provided excellent results in the flexibility and buckling resistance tests. These results suggested that the use of a tube having a layer formed of FEP, PTFE, or PE and another layer formed of FKM or silicon rubber would provide excellent results in each test. For verification, the above-described tests were conducted on a tube having an inner layer formed of PE and an outer layer formed of olefin rubber. In each test, excellent results were obtained. In addition, when the tapered portion of the joint36was press-fitted into such a double-layer tube, the tube provided a good seal around the tapered portion without being torn or broken.

In the double-layer tube5shown inFIG. 5, the inner layer50formed of FEP or other suitable materials prevents evaporation of moisture contained in the ink and air permeation through the tube5, and the outer layer51formed of silicon rubber or other suitable materials is flexible enough to provide flexibility and buckling resistance required by the ink-jet printer1. Additionally, the inner layer50is set to have a thickness D3of about 75 μm, which is not too thick to reduce flexibility of the tube5. The outer layer51is set to have a Shore A hardness of 70, which is just right for reducing the bending stress exerted on the inner layer50. In addition, the inner layer50formed of FEP and the outer layer51formed of silicon rubber can be firmly bonded to each other by treating the FEP surface using etchants, i.e., etching agents, such as TETRA-ETCH®. When the inner layer50and the outer layer51are respectively formed of olefin base resin and olefin base rubber, which are of the same type of material, the inner and outer layers50,51can be simultaneously formed by extrusion and firmly bonded to each other by melting.

Referring now toFIG. 6, an alternate form of the tube5will be described.FIG. 6is an enlarged cross-sectional view of an alternate tube105. The tube105is double-layered and has an inner layer52that contacts ink and an outer layer53bonded over the outer periphery of the inner layer52. The tube105has an inner diameter D1of about 1.4 mm and an outer diameter D2of about 3.0 mm. The inner layer52is formed of silicon rubber and has a Shore A hardness of about 70. The outer layer53is formed of FEP and has a thickness D4of about 75 μm. In short, the alternate tube105is formed by reversing the inner layer50and the outer layer51of the tube50shown in FIG.5. The tube105is as effective as the tube5in preventing vapor and air transmission through the tube105and reducing the bending stress exerted on the tube105. The above-described materials suitable for the inner and outer layers50,51can be used for the outer and inner layers53,52, respectively.

Referring now toFIGS. 7-10, a connection between the second joint36of the joint unit12and the tube5, shown inFIG. 2, will be described.FIGS. 7A and 7Bshow a first method of connecting the second joint36to the tube5. As described referring toFIG. 2, the second joint36has a neck36bprojecting from the body of the joint unit12and a bead36aextending from the neck36band tapered down toward the corresponding tube5a-5d. The head36ahas a maximum diameter larger than the inner diameter D1of the tube5, and the neck36bhas an smaller diameter than the maximum diameter of the head36a. The second joint36has a corresponding one of the ink paths12a-12dformed therein. The outer diameter d1of the tapered end of the head34ais about 1.3 mm, while the inner diameter D1of the tube5is about 1.4 mm. Thus, the outer diameter d1of the tapered end of the head36ais smaller than the inner diameter D1of the tube5by about 0.1 mm. When the second joint36is inserted into the tube5, a gap is created between the outer periphery of the tapered head36aand the inner periphery of the tube5.

FIG. 7Ashows a first method of connecting the second joint36to the tube5. Glycerin41is first applied to the tapered end of the head36a, at least to a portion having a smaller diameter than the inner diameter D1of the tube5. Then, as shown inFIG. 7B, the second joint36applied with glycerin is inserted into the tube5. Thereby, the second joint36is connected to the tube5while a gap created between the outer periphery of the tapered head36aand the inner periphery of the tube5is filled with glycerin. Accordingly, an ink ejection failure due to accumulation of air bubbles is prevented. Further, although a step is formed, because the velocity of the flow of ink past the step is fairly high, the accumulation of air bubbles is prevented. Alternatively, glycerin41may be applied to the inner periphery of the tube5.

FIGS. 8A-8Cshow a second method of connecting the second joint36to the tube5while eliminating air bubbles from their connection. The same elements as designated and described inFIGS. 7A and 7Bwill not be redundantly described.

As shown inFIG. 8A, the second joint36is first inserted into the tube5to establish a connection therebetween. In this case, because the outer diameter d1of the tapered end of the head36ais smaller than the inner diameter D1of the tube5, a gap42is created between the second joint36and the tube5. Then, as shown inFIG. 8B, the pressure inside the connected second joint36and tube5is reduced using a vacuum pump or the like to discharge air from the gap42. Then, as shown inFIG. 8C, glycerin41is supplied into the connected second joint36and tube5under the reduced pressure. Thereafter, the pressure is returned to an atmospheric pressure. Even after glycerin41is discharged from the ink path12a-12d, the gap42remains filled with glycerin41, as in FIG.7B. Accordingly, no air is trapped in the gap42, and an ink ejection failure due to accumulation of air bubbles is prevented.

FIGS. 9A-9Dshow a third method of connecting the second joint36to the tube5while eliminating air bubbles from their connection. The same elements as designated and described inFIGS. 7A and 7Bwill not be redundantly described.

As shown inFIG. 9A, the second joint36is first inserted into the tube5, and glycerin41is supplied into the connected second joint36and tube5. In this case, because the outer diameter d1of the tapered end of the head36ais smaller than the inner diameter D1of the tube5, a gap42is created between the second joint36and the tube5, and the gap is filled with air. Then, as shown inFIG. 9B, the pressure inside the connected second joint36and tube5is reduced to expand the air trapped in the gap42. Glycerin41remains unchanged because a liquid is uncompressive. Then, as shown inFIG. 9C, glycerin41is again supplied into the connected second joint36and tube5to discharge the expanded air with the velocity of flow of glycerin. When the pressure is returned to an atmospheric pressure, the gap42is filled with a small amount of compressed air and glycerin41. Even after glycerin41is discharged from the ink path12a-12d, glycerin41remains in the gap42, as in FIG.7B. Accordingly, an ink ejection failure due to accumulation of air bubbles is prevented.

InFIGS. 7-9, various filling liquids, including glycerin, which are used to fill printheads when shipped are commonly used as liquids to fill the gap. Specifically, a liquid obtained by removing a colorant and a volatile component from ink used for printheads is preferable as a filling liquid. Alternatively, ink actually used for printheads may be used.

FIGS. 10A and 10Bshow connections between the second joint36and the tube5, when the outer diameter d1of the tapered end of the head36ais set differently. The same elements as designated and described inFIGS. 7A and 7Bwill not be redundantly described.

InFIG. 10A, the outer diameter d1of the tapered end of the head36ais set to be about 1.4 mm, while the inner diameter D1of the tube5is set to be about 1.4 mm. In other words, the outer diameter d1of the tapered end of the head36ais equal to or slightly larger than the inner diameter D1of the tube5. Thus, when the second joint36is inserted into the tube5to establish a connection therebetween, no gap is created between the outer periphery of the tapered head36aand the inner periphery of the tube5. However, a step43is created at the tapered end of the head36aso as to face the flow of ink, due to the different inner diameters of the second joint36and the tube5. Because ink flows toward the step43, the velocity of flow of ink is fairly high, thus preventing accumulation of air bubbles at the step43and an ink ejection failure caused by accumulated air bubbles.

Alternatively, inFIG. 10B, the outer diameter d1of the tapered end of the head36ais set to be about 1.5 m, while the inner diameter D1of the tube5is set to be about 1.4 mm. In other words, the outer diameter d1of the tapered end of the head36ais larger than the inner diameter D1of the tube5by about 0.1 mm. Thus, when the second joint36is inserted into the tube5to establish a connection therebetween, a recess44is created in the ink path. Air bubbles are less likely to be trapped in such a recess44than in the gap42(FIG. 8A) created between the outer periphery of the tapered head36aand the inner periphery of the tube5. Accordingly, an ink ejection failure caused by air bubbles is prevented. As described referring toFIG. 5, because the tube5may be formed to have an inner diameter of between about 0.8-2.0 mm and an outer diameter of between about 2.4-4.0 mm, the head36ain the above-described exemplary methods may be dimensioned in proportion to the inner and outer diameters of the tube5.

FIG. 11shows a connection between the second joint36and the tube5additionally using a locking member37. The second joint36and the tube5are dimensioned similarly to those inFIG. 10A, and the head36aof the second joint36has, at its tapered end, an outer diameter d1of about 1.4 mm, which is substantially equal to or slightly larger than the inner diameter D1of the tube5. The locking member37is provided to lock the outer periphery of the tube5covering the second joint36.

The locking member37is formed as a substantially hollow cylinder, and has an inner diameter smaller than the outer diameter of a connection between the tube5and a maximum-diameter portion of the head36a. Thus, when the locking member37is fitted around the outer layer51of the tube5covering the second joint36, the locking member37presses the flexible outer layer51formed of silicon rubber to bring the tube5into more intimate contact with the second joint36. At this time, the locking member37extends over the outer layer51of the tube5generally from the tapered end of the head36ato the maximum-diameter portion of the head36a. Accordingly, even when the tube5moves randomly as the printhead unit3(carriage3a) reciprocates, the tube5and the second joint36, connected to each other, are unlikely to be loosened to permit the entry of air therebetween and unlikely to be detached from each other. Especially, silicon rubber is highly restorable and unlikely to be plastically deformed by the pressure from the locking member37, and thus intimate contact between the second joint36and the inner layer50of the tube5can be maintained for a long time.

Referring now toFIG. 12, a locking member137, as an alternate form of the locking member37inFIG. 11, will be described. The locking member137is longer than the locking member37in the axial direction. When the locking member37is fitted around the outer layer51of the tube5covering the second joint36, the locking member137presses the outer layer51over a longer length to fit the inner layer50tightly to the second joint36. At this time, the locking member137extends over the outer layer51of the tube5generally from the tapered end of the head36ato the middle of the neck36b. Accordingly, the long locking member137locks the interconnected tube5and second joint36more securely and prevents them from being loosened or detached from each other, even when the tube5and the second joint36expand or contract with changes in temperature.

Referring now toFIGS. 13A and 13B, a locking member237, as an alternate form of the locking member37inFIG. 11, will be described. As shown inFIG. 13B, the locking member237is substantially cylindrical and has higher rigidity than the tube5. The locking member237has a first inner-diameter portion237aand a second inner-diameter portion237b. The first inner-diameter portion237ahas a first inner diameter K1smaller than the outer diameter of the connection between the tube5and the maximum-diameter portion of the head36aof the second joint36. The second inner-diameter portion237bprojects radially inwardly and has a second inner diameter K2smaller than the first inner diameter K1. Further, slits237care formed from an end of the second inner-diameter portion237bto the first inner-diameter portion237ato divide the second inner-diameter portion237binto several segments, each having radial resilience. The tube5covering the second joint36is inserted into the locking member237, which in turn locks the interconnected tube5and second joint36.

Because the first inner diameter K1is smaller than the outer diameter of the connection between the tube5and the maximum-diameter portion of the second joint36, the first inner-diameter portion237aof the locking member237compresses the flexible outer layer51of the tube5, and the compressed tube5presses the second joint36. Thereby, the locking member237locks the connection between the tube5and the second joint36. In addition, the second inner-diameter portion237bwith slits237cis enlarged in inner diameter K2to allow the connection between the tube5and the maximum-diameter portion of the second joint36to be inserted into the second inner-diameter portion237b. The second inner-diameter portion237bpresses the tube5against the neck36bof the second joint36. Thus, the tube5radially expanded by the maximum-diameter portion is radially compressed toward the neck36b. This structure effectively prevents the tube5from being detached from the second joint36. In addition, because silicon rubber used for the outer layer51of the tube5is highly restorable and unlikely to be plastically deformed by the pressure from the locking member237, intimate contact between the second joint36and the inner layer50of the tube5can be maintained for a long time.

Referring now toFIG. 14, a locking member337as an alternate form of the locking member237will be described.FIG. 14shows the tube5, the second joint36, and the locking member337when they are connected. The same elements as designated and described inFIGS. 13A and 13Bwill not be redundantly described. The locking member337has a first inner-diameter portion337aand a second inner-diameter portion337bextending from one end of the first inner-diameter portion337a. The first and second inner-diameter portions337a,337bof the locking member337are similar to the first and second inner-diameter portions237a,237bof the locking member237. Additionally, the locking member337has a third inner-diameter portion337dthat projects radially inwardly from the other end of the first inner-diameter portion337aand has a third inner diameter K3substantially equal to the outer diameter D2of the tube5. When the tube5and the second joint36are locked by the locking member337in the same manner as inFIGS. 13A, the third inner-diameter portion337dis brought into contact with the outer periphery of the tube5, in a close vicinity to a contact portion39between the outer periphery of the tapered end of the second joint36and the inner layer50of the tube5.

The second joint36and the tube5are dimensioned similarly to those in FIG.10A. The inner diameter D1of the tube5is substantially equal to or slightly smaller than the outer diameter d1of the tapered end of the second joint36to prevent accumulation of air. Thus, when the second joint36is connected to the tube5, the outer periphery of the tapered end of the second joint36contacts the inner layer50of the tube5. Without the third inner-diameter portion337d, random movements of the tube5caused by the reciprocating printhead unit3(carriage3a) would exert stresses in the vicinity of the contact portion39between the second joint36and the tube5, and such stresses would cause a crack in the inner layer50formed of a hard material, such as fluorinated ethylene propylene (FEP).

However, because the third inner-diameter portion237dis provided on the outer layer51of the tube5, on the opposite side of the contact portion39from the second joint36, random movements of the tube5will exert stresses at a contact portion40between the third inner-diameter portion337dand the outer layer51of the tube5. The outer layer51of the tube5formed of silicon rubber absorbs such stresses with the resiliency of the silicon rubber.

As described above, by the use of the locking member37,137,237,337the interconnected tube5and second joint36are firmly locked. Accordingly, even when the tube5moves randomly as the printhead unit3(carriage3a) reciprocates, the locking member37,137,237,337prevents the tube5from being detached from the second joint36.

Although the above-described locking member37,137,237,337is formed into a substantially hollow cylinder, the locking member37,137,237,337may take various forms. For example, the inner periphery of the locking member37,137,237,337may be tapered so as to follow the contour of the outer periphery of the head36a. In this case, the tube5is locked more firmly by the locking member37,137,237,337and the head36a. Alternatively, the locking member37,137,237,337may be formed into a belt to be wrapped around the connection between the tube5and the second joint36.

Referring now toFIG. 15, a connection between the tube105shown inFIG. 6 and asecond joint136that has a longer neck136bthan the second joint36will be described. As shown inFIG. 15, the second joint136has a maximum diameter d4larger than the inner diameter D1of the tube105. The second joint136has a head136atapered down toward its open end and a neck136bthat extends from the maximum-diameter portion and has a smaller diameter than the maximum-diameter portion. The outer diameter d1of the tapered end of the head136ais about 1.4 mm, which is substantially equal to or slightly larger than the inner diameter D1(about 1.4 mm) of the tube105. The maximum diameter d4is about 2.5 mm. The neck136bhas a length d2of about 2.5 mm and an outer diameter d3of about 1.6 mm. However, the tube105may variably sized to have an inner diameter of between about 0.8-2.0 mm and an outer diameter of between about 2.4-4.0 mm, and the second joint136may be dimensioned in proportion to the inner and outer diameters of the tube105. The tube105and the second joint136are interconnected by inserting the head136aand the neck136bof the second joint136into the tube5.

The outer diameter d1of the tapered end of the head136ais substantially equal to or slightly larger than the inner diameter D1of the tube105. This allows the second joint136to be inserted fairly easily into the tube105and to be connected to the tube105without a gap created between the outer periphery of the tapered head136aand the inner periphery of the inner layer of the tube105. The absence of a gap prevents air accumulation and clogging of the ink path with accumulated air bubbles.

The head136aof the second joint136has the maximum diameter d4larger than the inner diameter D1of the tube105and is tapered down toward its open end. Because the inner layer52of the tube105is formed of flexible silicon rubber, the tube105is gradually radially expanded by the tapered head136aand expanded most at its maximum-diameter portion. Thus, the inner periphery of the tube105closely contacts the outer periphery of the head136b, thereby preventing the entry of air between the tube105and the second joint136.

In addition, the length d2of the neck136bis about 2.5 mm, and the difference between the outer diameter d3of the neck136band the maximum diameter d4of the head36ais about 0.9 mm. If a force pulling the tube105out of the second joint136is applied to the tube105, the second joint136thus dimensioned provides a sufficient resistance against expansion of the end of the tube105toward the head136a. Accordingly, the tube is hardly loosened or detached from the second joint136when the printhead unit3(carriage3a) reciprocates.

According to experiments carried out by the inventor, the length d2of the neck136bis preferably about 0.7 or more times, and more preferably about 1.5-2.0 times, the inner diameter D1of the tube105, considering the ease of insertion of the second joint136into the tube105and the strength of the second joint136. Half the difference between the outer diameter d3of the neck136band the maximum diameter d4of the head136a, which corresponds to the radial length of a step formed by the outer periphery of the neck36band the maximum-diameter portion of the head136a, is preferably substantially equal to or greater than about 0.3 times the inner diameter D1of the tube105.

According to the above-described ink-jet printer1, the ink path, formed by connecting the first joint35,135and the joint34of the air trap unit11and by connecting the second joint36,136and the tube5,105, is substantially free of air-trapping gaps. Accordingly, clogging of the ink path with accumulated air is unlikely to occur, and, thus, good ink ejection and high print quality are ensured. In addition, the tube5,105is double-layered and has a layer formed of a flexible material and another layer formed of a material with low vapor and gas permeability. Accordingly, the tube5,105is resistant to buckling, flexible enough to provide an air-tight seal around the mating joint, and able to prevent evaporation of moisture contained in the ink and air permeation therethrough.

In the above-described connections between the second joint36(FIGS. 7-14) and the tube5(FIG. 5) and between the second joint136(FIG. 15) and the tube105(FIG.6), the tubes5,105may be interchangeably used. In other words, either of the tubes5,105that have a layer formed of a flexible material and another layer formed of a material with low vapor and air permeability may be used, regardless of which layer is the inner or outer layer.

Further, the connecting structure between the joint34and the first joint35,135, and the connecting structure between the tube5,105and the second joint36,136may be interchangeably used to connect the joint unit12and the air trap unit11and to connect the joint unit12and the ink source.

Although the invention has been described with reference to a specific embodiment, the description of the embodiment is illustrative only and is not to be construed as limiting the scope of the invention. Various other modifications and changes may occur to those skilled in the art without departing from the spirit and scope of the invention.