Patent Publication Number: US-6217164-B1

Title: Ink jet recorder

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an ink jet recorder. In particular, the invention relates to an ink jet recorder with an ink supply system including an ink tank and a recording head which are connected by a supply tube. 
     2. Description of Related Art 
     A conventional ink jet recorder such as an ink jet printer includes an ink tank and a recording head, which are connected by a supply tube and a return tube. The recorder also includes a pump for circulating ink between the tank and the head. 
     FIGS. 27-29 of the accompanying drawings schematically show a known ink supply system. With reference to FIG. 27, an ink jet printer  100  includes a sub tank  4   a  and a head unit, which includes a print head  5   a . The tank  4   a  and the head  5   a  are connected by two supply tubes  21   a  and two return tubes  22   a . The upstream supply tube  21   a  is fitted with a circulating pump. The upstream supply tube  21   a  and the downstream return tube  22   a  are connected to the downstream supply tube  21   a  and the upstream return tube  22   a , respectively, by a joint  40 . The joint  40  has two passages formed through it, one of which communicates with the supply tubes  21   a , and the other passage communicates with the return tubes  22   a.    
     Each of the passages is fitted with a filter  32  for removing foreign substances etc. from ink. The head unit is mounted on a carriage, and can reciprocate at high speed over a printing medium. The reciprocating unit is followed by the upstream supply tube  21   a  and the downstream return tube  22   a , which are connected to it. While these tubes  21   a  and  22   a  are reciprocating at high speed, waves are generated in the ink in the tubes. If the waves propagate to the head  5   a , they affect the characteristics of ink ejection. The filters  32  for the passages also function to prevent the waves from propagating to the head  5   a . The filters  32  are positioned horizontally so that ink can flow vertically through them. The ink flowing between the supply tubes  21   a  moves downward through the associated filter  32 . The ink flowing between there turn tubes  22   a  moves upward through the other filter  32 . 
     Otherwise, the joint  40  and the filters  32  might be positioned in such a manner that ink could flow horizontally through the filters. 
     Very small air bubbles may be contained in the ink supplied from the sub tank  4   a  to the recording head  5   a . If some of the bubbles are trapped on the filter  32  in the supply line, the trapped bubbles tend to move up buoyantly, and can hardly pass through this filter. Therefore, the trapped bubbles are liable to stay or be retained on the filter  42 . The staying bubbles reduces the contact or touch area between the filter  42  and ink, and therefore reduces the effective area of this filter. This prevents a sufficient amount of ink from flowing to the head  5   a , and therefore defective recording may occur. The staying bubbles act as dampers, which may hinder or impede the intended normal ink flow when the circulating pump is driven. 
     As shown in FIGS. 28 and 29, the joint  40  includes a double connector  151 , a seal member  152  and a holder  153 , which engages with the connector. The double connector  151  includes integral twin connectors, each of which is connected to one of the upstream supply tube  21   a  and the downstream return tube  22   a . Each of the twin connectors is fitted with one of the filters  32  on its bottom. The seal member  152  surrounds bottom portions of the twin connectors. The holder  153  includes integral twin connectors, each of which is connected to one of the downstream supply tube  21   a  and the upstream return tube  22   a . The double connector  151  also includes lock pawls  151   a . The holder  153  also includes lock pawls  153   a  for each engaging with one of the pawls  151   a  to keep it and the double connector  151  connected. The holder  153  further includes a ridge  153   b  formed on its top for engaging with the seal member  152  to seal the junctions between the holder  153  and the connector  151 . The ridge  153   b  is positioned away from the passages in the joint  40  and the filters  32 . Therefore, gaps are liable to be formed between the ridge  153   b  and each of the passages and the filters  32 . Air bubbles may stay in the gaps. If the bubbles grow larger, they narrow the passages. This makes it impossible to secure the necessary amount of ink flow. 
     Because the joint  40  includes a number of parts, its production costs are high. 
     Another conventional ink jet recorder of the foregoing type includes a carriage, on which a head holder can be mounted. The head holder is fitted with a connector. The system also includes an ink tank, which is connected to a supply tube and a return tube. When the head holder is mounted on the carriage, the connector is connected to the tubes. The mounting involves positioning the head holder with high accuracy to align the connector with the tubes. 
     U.S. Pat. No. 5,359,357, which corresponds to Japanese Patent Application Laid-Open No. 5-318761, discloses an ink jet recorder including a carriage, a head unit, an ink supply tube and an ink return tube. Each of the tubes is connected to a connector. The elasticity of the seal rubber of the connector absorbs the displacement or misalignment of the connector from the associated tube when the head unit is mounted on the carriage. Because the amount of deformation of the seal rubber is limited, however, it is not possible to absorb a large amount of displacement. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing problems, it is an object of the invention to provide an ink supply system for an ink jet recorder in which, when ink flows through the filter in the ink passage, the air trap at the filter is reduced to increase the substantially effective area of the filter. 
     It is another object of the invention to provide an ink supply system for an ink jet recorder in which common parts are used for the joint connecting the ink supply tubes and for the joint connecting the ink return tubes so that the number of parts and the production costs for the joints can be reduced, and in which no air bubbles stay in the joints so that good recording results can be produced. 
     It is still another object of the invention to provide an ink supply system for an ink jet recorder which includes small joints in the ink supply lines, the joints being able to be assembled out of parts by a simple process, and in which few air bubbles are liable to stay at the junctions between parts of the joints so that ink can be supplied always normally. 
     It is a further object of the invention to provide an ink supply system for an ink jet recorder in which, when the recording head is mounted on the carriage, it is possible to simply align the upstream and downstream supply tubes with each other. 
     In accordance with a first aspect of the invention, an ink jet recorder is provided which comprises: 
     a tank for storing ink; 
     an ink jet head for ejecting the ink onto a recording medium; 
     an ink tube for connecting the tank and the head, a part of the ink tube being formed such that the ink can flow upward; and 
     an ink filter for removing foreign substances from the ink, the ink filter being fitted in the part of the ink tube. 
     The ink filter of this recorder is fitted in the part of the ink tube where ink can flow upward. Consequently, the air bubbles flowing with ink from the tank or the air bubbles produced in the ink pass easily with buoyancy through the filter, and are therefore restrained from being trapped by the filter. This prevents the bubbles from staying at the filter, and can therefore increase the effective area of the filter. 
     In general, an ink jet recorder carries out cleaning or maintenance by ejecting ink forcedly out of the ink jet head to prevent the head from clogging. The maintenance involves discharging air bubbles together with ink from the head, or recovering air bubbles into the tank. 
     The recorder according to the first aspect may include a pump for circulating ink between the tank and the recording head. The pump can send ink forcedly to the head. The sent ink flows upward through the filter. Therefore, during the maintenance as well, air bubbles pass easily through the filter. This results in good maintenance. The ink tube of this recorder may include a supply tube for supplying ink from the tank to the head and a return tube for returning ink from the head to the tank. The pump can circulate ink through the tubes. 
     The filter of this recorder may include a first filter and a second filter. The supply tube may include a part where ink can flow upward. The first filter is fitted in this tube part. The return tube may include a part where ink can flow upward. The second filter is fitted in this tube part. This enables the filter to remove foreign substance effectively in both of the tubes. 
     The ink jet head may be mounted on a carriage, which can move the head over the recording medium. In this case, a first joint may be fitted in that part of the supply tube where ink can flow upward. This joint has an ink passage formed therein. The first filter is fitted in the passage. When the head is removed from the carriage, the supply tube can be detached from the joint. By fitting the filter in the joint, it is easy to mount the filter and assemble the ink flow system of the recorder. 
     That part of the return tube where ink can flow upward may be the part connecting the ink jet head and the return tube. It is easy to control the ink flow in this connecting part in such a manner that ink flows upward. A second joint may be fitted in this part of the return tube, and has an ink passage formed therein. The second filter is fitted in this passage. This makes it easy to mount the filter in the return tube and assemble the ink flow system of the recorder. 
     The supply tube may be formed such that the ink can flow upward. The return tube may may be formed such that the ink can flow upward. An integral joint may be fitted in these parts. This joint may have a first passage and a second passage formed therein, and communicating with the supply and return tubes, respectively. The filter may include a plane filter crossing the passages. The filter may be positioned in the joint in such a manner that the ink flowing through the passages moves obliquely through the filter. In this case, because ink can pass through the filter inclined with respect to its flow, the contact area between the filter and ink widens. This makes it possible to remove foreign substances efficiently. In order that the filter may incline with respect to the ink flow, the first and second passages may be bent with respect to the ink flow through the supply and return tubes, respectively. 
     The ink jet head of this recorder may have an array of ink channels formed therein. The tank of this recorder may be a detachable ink cartridge. The invention may also be applied to a recorder with which an ink cartridge is used. 
     In accordance with a second aspect of the invention, an ink jet recorder is provided which comprises: 
     a tank for storing ink; 
     an ink jet head for ejecting the ink; 
     a supply tube for supplying the ink from the tank to the head, the supply tube having an upstream supply tube connected to the tank and a downstream supply tube connected to the head; 
     a return tube for returning the ink from the head to the tank, the return tube having an upstream return tube connected to the head and a downstream return tube connected to the tank; 
     a pump for circulating the ink through the tubes between the tank and the head; 
     a first joint for connecting the upstream and downstream supply tubes; 
     a second joint for connecting the upstream and downstream return tubes; 
     each of the joints including a seal member having a passage formed therethrough, such that the passage widens toward one open end of the seal member; 
     one of the joints including a first connector having an open end for connection to the open end of the seal member, the first connector having a passage formed in the open end thereof such that the connector passage widens toward the open end of the first connector; and 
     a filter fitted in one of the open ends of the first connector and the associated seal member; 
     the other joint including a second connector having a convex part shaped for engagement with the passage in the associated seal member, the second connector having a passage formed therein. 
     In accordance with a third aspect of the invention, an ink jet recorder is provided which comprises: 
     a tank for storing ink; 
     an ink jet head for ejecting the ink; 
     a supply tube for supplying the ink from the tank to the head, the supply tube having an upstream supply tube connected to the tank and a downstream supply tube connected to the head; 
     a return tube for returning the ink from the head to the tank, the return tube having an upstream return tube connected to the head and a downstream return tube connected to the tank; 
     a pump for circulating the ink through the tubes between the tank and the head; 
     a first joint for connecting the upstream and downstream supply tubes; 
     a second joint for connecting the upstream and downstream return tubes; 
     one of the joints including a first seal member and a first connector; 
     the first seal member having a passage formed therethrough such that the passage widens toward one open end of the first seal member; 
     the first connector having an open end for connection to the open end of the first seal member, the first connector having a passage formed in the open end thereof such that the first connector passage widens toward the open end of the first connector; and 
     a filter fitted in one of the open ends of the first connector and the first seal member; 
     the other joint including a second connector and a second seal member; 
     the second connector being substantially identical in shape with the first connector, the second connector having a passage formed therein such that the second connector passage widenes toward the open end of the second connector; 
     the second seal member having a convex open end shaped for engagement with the second connector passage, the second seal member having a passage formed therein. 
     In the recorder according to the second or third aspect, each of the joints includes a connector and a seal member. One of the joints is fitted with a filter, and the other may be fitted with no filter. The connectors or the seal members of the joints may be identical in structure. This makes the connectors or the seal members common to the joints, reducing the production costs for the recorder. 
     In the recorder according to the second or third aspect, the joint fitted with the filter and the associated tubes may be positioned in such a manner that ink can flow upward through the filter. This causes air bubbles to flow easily through the filter, increasing its effective area. The passages widening outward may be conical, and each of the convex part and end may be shaped for compressive contact with the conical surface of the associated passage. This makes it possible to engage each of the connectors securely with the associated seal member. The connectors of the joints may be integral. This makes the parts of the joints fewer. This also makes it possible to engage the connectors with the seal members in one step. The joints may be fitted to a head unit, on which the recording head can be mounted. 
     In accordance with a fourth aspect of the invention, an ink jet recorder is provided which comprises: 
     an ink tank; 
     an ink jet head; 
     a first tube connected to the head; 
     a second tube connected to the tank; 
     a connecting member for connecting the first and second tubes, the connecting member including a first connector and a second connector each having a passage formed therein, the passage having a conical open end for connection to the conical open end of the passage in the other connector; and 
     a filter interposed between the connectors; 
     the second connector being made of sealing material. 
     The structure of this connecting member needs no such special seal member in conventional printer as shown in FIGS. 28 and 29. The inventive connection member can reduce the number of parts and the production costs for the recorder. The connecting member may be so positioned that ink can flow upward through the filter. 
     This recorder may further comprise a head holder, which holds the ink jet head, and a mounting member for mounting the connectors on the holder. The second connector may be a seal member. By engaging the first connector with the mounting member, it is possible to keep this connector and the seal member connected. Because the engagement of the first connector with the mounting member connects this connector and the seal member on the head holder, the connection can be maintained securely. 
     Each of the first and second tubes of this recorder may include a supply tube and a return tube for circulating ink between the ink tank and the ink jet head. The connecting member of this recorder may include first and second connecting members, each of which includes the first and second connectors. The first connecting member can connect the supply tubes of the first and second tubes. The second connecting member can connect the return tubes of the first and second tubes. 
     The first connectors of the two connecting members may be integral with each other. The seal members of the two connecting members may be integral with each other. This makes it possible to engage the first connectors with the seal members at a time for the supply and return tubes. It is therefore possible to simplify the process for engaging parts, and improve the productivity. 
     Each of the first connector and the seal member of each connecting member may have an end surface for contact with the end surface of the other. The end surface of one of the first connector and the seal member of each connecting member may have a ridge formed thereon for sealing between the end surfaces of the first connector and the seal member. This can shorten the distance between the ridge and the associated filter, and the distance between the ridge and ink. Therefore, gaps are hardly or difficultly produced between the ridge and the filter, and between the ridge and ink. Consequently, few air bubbles are liable to stay in the connecting member. 
     In accordance with a fifth aspect of the invention, an ink jet recorder is provided which comprises: an ink tank; 
     a head holder; 
     an ink jet head mounted on the holder for ejecting ink onto a recording medium; 
     a carriage for mounting the holder thereon and moving the head over the medium; 
     a first tube connected to the head; 
     a second tube connected to the tank; 
     a connecting member for connecting the first and second tubes; and 
     a mounting member for mounting the connecting member on one of the holder and the carriage; 
     a retainer formed on one of the head holder and the carriage so that the retainer retains the mounting member with play. 
     The mounting member of this recorder enables the connecting member to be mounted on the head holder or the carriage with a little play. This makes it possible to change the position of the connecting member relative to the holder or the carriage a little within the range of the play. 
     The first tube of this recorder may be fitted to the head holder. The second tube may be fitted to the carriage. The two fitted tubes may be displaced or misaligned (“s” in FIG.  11 ). Even in this case, when the holder is mounted on the carriage, the connecting member can be positioned within the range of the play between the connecting member and the retainer formed on the head holder or the carriage to absorb or compensate the displacement between the tubes. This can connect the two tubes securely. 
     The connecting member of this recorder may be fitted with a filter therein. As is the case with the recorder according to the first aspect, the connecting member, the filter and/or the tube may be positioned in such a manner that ink can flow upward through the filter. 
     The mounting member can be retained in the inner side of the retainer with play which is substantially perpendicular to the axis of the connecting member. One or both of the mounting member and the retainer may include a protrusion formed thereon for preventing the mounting member from falling axially of the connecting member. The protrusion keeps the mounting member from falling axially of the connecting member, and allows the mounting member to move substantially radially of the connecting member. This keeps the connecting member securely from falling, and allows it to move laterally. 
     The protrusion may be made of material which can elastically deform in such directions as to allow the mounting member to be retained within the retainer. The elastically deformable protrusion makes it easy to engage the mounting member with and disengage from the retainer. 
     The connecting member may include a first connector connected to the first tube and a second connector connected to the second tube. The first and second connectors are fitted to the mounting member, such as a joint holder, and the carriage, respectively as shown in FIG.  11 . The connectors can connect the two tubes easily and securely, and make it easy to make the head holder detachable from the carriage. 
     Each of the first and second tubes may include a supply tube and a return tube for circulating ink between the ink tank and the ink jet head. The connecting member may include first and second connecting members, each of which includes the first and second connectors. The first connecting member can connect the supply tubes of the two tubes. The second connecting member can connect the return tubes of the first and second tubes. Part of the ink supplied through the supply tubes to the head can return through the return tubes to the tank. 
     Each of the first and second connecting members may also include a seal member interposed between the first and second connectors to connect the first and second connectors. The first connector of each connecting member may be connected to the supply tube or the return tube of the first tube. The second connector of each connecting member may be connected to the supply tube or the return tube of the second tube. Because the seal member can connect the associated connectors, they can be connected securely. 
     The first and second connectors may be fitted to the head holder and the carriage, respectively. One or both of the connectors of each connecting member may be fitted with a little play. The play makes it possible to connect the connectors easily and securely even if they are displaced from each other. 
     This recorder is suitable as an ink jet recorder of the type in which the ink tank is fixed in the recorder. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the invention will be described with reference to the accompanying drawings, in which: 
     FIG. 1 is a schematic view of part of an ink jet recorder according to a first embodiment of the invention; 
     FIG. 2 is a schematic view showing the ink supply system for one of the recording heads of this recorder; 
     FIG. 3 is a schematic view showing the ink supply system for one of the recording heads of an ink jet recorder according to a second embodiment of the invention; 
     FIG. 4 is an exploded perspective view of the connectors and the filter of this supply system; 
     FIG. 5 is an exploded vertical section of these connectors and this filter; 
     FIG. 6 is a top view of the lower connector of this supply system; 
     FIG. 7 is an exploded vertical section of the head unit of an ink jet recorder according to a third embodiment of the invention; 
     FIG. 8 is a vertical section showing the step of mounting an ink cartridge on this head unit; 
     FIG. 9 is a vertical section showing this cartridge mounted on this head unit; 
     FIG. 10 is a schematic view showing the ink supply system for one of the recording heads of an ink jet recorder according to a fourth embodiment of the invention, 
     FIG. 11 is a vertical section showing the joints of this supply system; 
     FIG. 12 is a vertical section showing these joints in engaged condition; 
     FIG. 13 is an exploded perspective view of these joints etc.; 
     FIG. 14 is a vertical section showing joints according to a fifth embodiment of the invention; 
     FIG. 15 is a vertical section showing these joints in engaged condition; 
     FIG. 16 is a vertical section showing joints according to a sixth embodiment of the invention; 
     FIG. 17 is a vertical section showing joints according to a seventh embodiment of the invention; 
     FIG. 18 is a vertical section showing these joints in engaged condition; 
     FIG. 19 is a vertical section showing joints according to an eighth embodiment of the invention; 
     FIG. 20 is a vertical section showing these joints in engaged condition; 
     FIG. 21 is a vertical section showing a joint according to a ninth embodiment of the invention; 
     FIG. 22 is a vertical section showing a joint according to a tenth embodiment of the invention; 
     FIG. 23 is a vertical section showing the joint of the tenth embodiment in engaged condition; 
     FIG. 24 is a vertical section showing a joint according to an eleventh embodiment of the invention; 
     FIG. 25 is a vertical section showing the joint of the eleventh embodiment in engaged condition; 
     FIG. 26 is a schematic view showing the ink supply system for one of the recording heads of an ink jet recorder according to another embodiment of the invention; 
     FIG. 27 is a schematic view showing the ink supply system of a conventional ink jet recorder; 
     FIG. 28 is a vertical section showing the joint of this supply system; 
     FIG. 29 is a vertical section showing this joint in engaged condition. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     With reference to FIG. 1, an ink jet color recorder  1  according to a first embodiment of the invention includes a frame (not shown), which supports a horizontal feed roller  13 . The roller  13  can be turned by a drive mechanism (not shown), which includes a feed motor, to feed recording paper P perpendicularly to the roller axis. The frame also supports a guide bar  8  and a guide rail  9 , which extend in parallel with the feed roller  13 . A carriage  6  is supported slidably on the bar  8  and the rail  9 , and can be reciprocated along them by a timing belt  10 , which can be driven by a carriage motor  11 . A head unit  2  for color recording is mounted removably on the carriage  6 , and can scan the paper P as the carriage is reciprocated. The head unit  2  includes four recording heads  5   a - 5   d  for ejecting ink onto the paper P. 
     While the recording heads  5   a - 5   d  are used, air bubbles may be produced in them, and ink droplets may stick to their ejection faces. This may cause defective ejection of ink. Positioned on one side of the feed roller  13  is a maintenance unit, which includes a purging device  15  and a wiper  16  for recovering normal ejection of ink. The purging device  15  includes a suction cap  15   a  for close contact with the ejection face of one of the heads  5   a - 5   d  at a time. The cap  15   a  is connected to a pump  15   b , which can develop a negative pressure for sucking the bubbles in the head  5   a ,  5   b ,  5   c  or  5   d  capped with the cap, bad ink, etc. The sucked ink is sent to a reservoir  15   c . The wiper  16  can wipe away the ink dregs or sediment, etc. stuck to the ejection faces of the heads  5   a - 5   d . Positioned on one side of the purging device  15  is a capping device  17  for preventing the heads  5   a - 5   d  from drying while they are not used. 
     The recorder  1  also includes four main tanks  3   a - 3   d  filled with ink for supply to the head unit  2 . The recorder  1  further includes four sub tanks  4   a - 4   d  for temporarily storing the ink from the main tanks  3   a - 3   d , respectively. Ink can be circulated between the head unit  2  and the sub tanks  4   a - 4   d . The recording heads  5   a - 5   d  can be supplied with ink from the main tanks  3   a - 3   d  and the sub tanks  4   a - 4   d , respectively. The main tanks  3   a - 3   d  are connected to the sub tanks  4   a - 4   d  by tubes  19   a - 19   d , respectively. The tubes  19   a - 19   d  are fitted with pumps  20   a - 20   d  for supplying ink from the main tanks  3   a - 3   d , respectively. 
     FIG. 2 shows the ink supply system for the recording head  5   a . The heads  5   a - 5   d  and the ink supply systems for them are identical or similar in structure. 
     The sub tanks  4   a - 4   d  are connected to the recording heads  5   a - 5   d  by pairs of flexible supply tubes  21   a - 21   d  and pairs of flexible return tubes  22   a - 22   d , respectively. The upstream supply tube  21   a - 21   d  are fitted with pumps  23   a - 23   d  for supplying ink from the sub tanks  4   a - 4   d , respectively. Each of the pumps  23   a - 23   d  includes four rollers, which can revolve around its axis to squeeze the associated upstream supply tube  21   a ,  21   b ,  21   c  or  21   d . The pumps  23   a - 23   d  might be replaced with pumps of another known type. The downstream return tubes  22   a - 22   d  are fitted with valves  29   a - 29   d.    
     The interior of each of the sub tanks  4   a  has partitions or walls defining a labyrinth or maze of passages. Ink can flow from each of the main tanks  3   a - 3   d  and the associated downstream return tubes  22   a ,  22   b ,  22   c  or  22   d  into the associated sub tank  4   a ,  4   b ,  4   c  or  4   d , where the ink flows through the maze before it is supplied to the associated upstream supply tube  21   a ,  21   b ,  21   c  or  21   d . While the ink is flowing through the maze, air bubbles in the ink can separate buoyantly from it to be discharged into the atmosphere. 
     Each of the recording heads  5   a - 5   d  includes an actuator  25 , a nozzle plate  26  and a manifold  27 . The actuator  25  has an array of ink channels formed in it. The plate  26  has nozzles formed through it and each communicating with one of the channels. The manifold  27  can distribute the ink from the associated sub tank  4   a ,  4   b ,  4   c  or  4   d  to the channels, and is connected to the associated downstream supply tube  21   a ,  21   b ,  21   c  or  21   d  and upstream return tube  22   a ,  22   b ,  22   c  or  22   d . The carriage  6  is fitted with joints  304  and  314 , which are positioned nearby. Each of the upstream supply tubes  21   a - 21   d  are connected to the associated downstream supply tube by one of the joints  304 . Each of the upstream return tubes  22   a - 22   d  are connected to the associated downstream return tube by one of the joints  314 . 
     Each of the joints  304  for the supply tubes  21   a - 21   d  consists of a lower connector  305  and an upper connector  306 , which are connected to the associated downstream and upstream supply tubes, respectively. The connectors  306  and  305  for each of the supply tubes  21   a - 21   d  have conical bores  304   a  and  305   a  formed therein, respectively. The bore  304   a  or  305   a  of each of the connectors  306  and  305  widens or diverges toward its outer end, which can come into contact with the outer end of the other connector. Each of the upper connectors  306  is fitted with a filter  32  for removing foreign substances etc. from the ink flowing through it. The upper connectors  306  are fixed to the bottom of the head unit  2 . The lower connectors  305  are fixed to the top of the carriage  6 . As a result, the outlets of the upstream supply tubes  21   a - 21   d  are positioned in the carriage  6 , while the inlets of the downstream supply tubes  21   a - 21   d  are positioned above the carriage  6 . Therefore, when ink flows through the filters  32  of the upper connectors  306 , it moves upward. 
     Each of the upstream return tubes  22   a - 22   d  is connected to the associated manifold  27  by a connector  35 , which is fixed to the tube. The connector  35  has a conical bore formed in it and widening toward its outer end. The connector  35  is fitted with a filter  36  for removing foreign substances etc. from the ink flowing through it. The manifold  27  includes an outlet port  27   a  formed at its top. The outer end of the port  27   a  faces upward, and engages with the connector  35 . The port  27   a  has a bore formed in it and widening toward its outer end. When ink flows from the manifold  27  to the associated upstream return tube  22   a ,  22   b ,  22   c  or  22   d , it moves upward through the filter  36 . 
     Ink can flow in the ink circulation system as follows. 
     When each of the valves  29   a - 29   d  is opened and the associated pump  23   a ,  23   b ,  23   c  or  23   d  is driven, the associated manifold  27  is filled with ink from the associated sub tank  4   a ,  4   b ,  4   c  or  4   d  through the associated supply tubes  21   a ,  21   b ,  21   c  or  21   d . The surplus ink returns through the associated return tubes  22   a ,  22   b ,  22   c  or  22   d  to the tank  4   a ,  4   b ,  4   c  or  4   d . While the manifold  27  is filled with ink, ink circulates through the filters  32  and  36  between the tank  4   a ,  4   b ,  4   c  or  4   d  and the associated recording head  5   a ,  5   b ,  5   c  or  5   d . When the pump  15   b  of the purging device  15  is driven with the suction cap  15   a  in contact with the nozzle plate  26  of the head  5   a ,  5   b ,  5   c  or  5   d , ink is sucked from the manifold  27  and filled into the channels in the head. Thereafter, the pump  23   a ,  23   b ,  23   c  or  23   d  is stopped in such a position that the upstream supply tubes  21   a ,  21   b ,  21   c  or  21   d  is not choked, and the associated valve  29   a ,  29   b ,  29   c  or  29   d  is closed. As ink is ejected for recording from the head  5   a ,  5   b ,  5   c  or  5   d , the manifold  27  is replenished with ink from the tank  4   a ,  4   b ,  4   c  or  4   d  through the supply tubes  21   a ,  21   b ,  21   c  or  21   d . Air bubbles in the ink in the circulation line may gradually incorporate together and grow larger, narrowing or blocking the ink passages. This can be prevented by a periodic or manual operation which includes opening the valve  29   a ,  29   b ,  29   c  or  29   d  and driving the pump  23   a ,  23   b ,  23   c  or  23   d  to circulate ink between the tank  4   a ,  4   b ,  4   c  or  4   d  and the head  5   a ,  5   b ,  5   c  or  5   d  as stated above. The circulation recovers air bubbles from the supply tubes  21   a ,  21   b ,  21   c  or  21   d , the manifold  27  and the return tubes  22   a ,  22   b ,  22   c  or  22   d  into the tank  4   a ,  4   b ,  4   c  or  4   d , where the recovered bubbles are separated buoyantly from the ink while the ink is flowing through the maze of passages in the tank as stated above. As a result, the manifold  27  is supplied with ink containing a reduced number of air bubbles through the supply tubes  21   a ,  21   b ,  21   c  or  21   d.    
     Each of the sub tanks  4   a - 4   d  is fitted with a level sensor (not shown), which detects the amount of ink stored in the tank. On the basis of the value detected by the sensor, the associated pump  20   a ,  20   b ,  20   c  or  20   d  (FIG. 1) is controlled to supply the associated sub tank  4   a ,  4   b ,  4   c  or  4   d  with ink from the associated main tank  3   a ,  3   b ,  3   c  or  3   d  so that a predetermined amount of ink is maintained always in the sub tank. 
     Thus, when ink flows through the filters  32  and  36  of the connectors  304  and  35 , it moves upward. Therefore, air bubbles flowing together with ink upward to the filters  32  and  36  pass through them easily with buoyancy, and are hardly trapped by them. This prevents air bubbles from staying at the filters  32  and  36 . Consequently, the effective areas of the filters  32  and  36  increase substantially. It is therefore possible to secure the quantity of ink flow which is necessary for recording. As a result, defective recording occurs hardly When ink circulates through the filters  32  and  36 , and/or when the purging device  15  sucks ink, no damper acts at the filters. It is therefore possible to realize a smooth air flow. 
     The recorder  1  may be modified in various ways. Each of the filters  32  might be fitted to the associated lower connector  305  in place of the associated upper connector  306 , or interposed between them. Each of the filters  36  might be fitted to the outlet port  27   a  of the associated manifold  27  in place of the connector  35 . The filters  32  and  36  might be positioned elsewhere in the recorder  1 , as far as they are supported in such a manner that ink can flow upward through them. 
     FIG. 3 shows the ink supply system for one of the recording heads  5   a - 5   d  (only  5   a  shown) of an ink jet color recorder  1  according to a second embodiment of the invention. 
     Each of these recording heads  5   a - 5   d  includes a manifold  27 , which is connected directly to a downstream supply tube  21   a ,  21   b ,  21   c  or  21   d  and an upstream return tube  22   a ,  22   b ,  22   c  or  22   d . The recorder  1  includes sub tanks  4   a - 4   d , which are connected to upstream supply tubes  21   a - 21   d  and downstream return tubes  22   a - 22   d , respectively. The upstream supply tubes  21   a - 21   d  and the downstream return tubes  22   a - 22   d  are connected to the downstream supply tubes  21   a - 21   d  and the upstream return tubes  22   a - 22   d , respectively, by upper connectors  40  and lower connectors  41 . In FIG. 3, the upstream supply tube  21   a , the downstream return tube  22   a  and the associated connectors  40  and  41  are shown as disconnected. 
     With reference to FIGS. 4-6, each of the connectors  40  and  41  has a wider groove and a narrower groove formed in it. Each upper connector  40  includes outlet pipes  40   a  and  40   b  formed near both its ends. The pipes  40   a  and  40   b  communicate with the adjacent ends of the narrower and wider grooves, respectively, of the upper connector  40 . Each lower connector  41  includes inlet pipes  41   a  and  41   b  formed near both its ends. The pipes  41   a  and  41   b  communicate with the adjacent ends of the narrower and wider grooves, respectively, of the lower connector  41 . The inlet pipe  41   b  and the outlet pipe  40   b , which communicate with the wider groove, are connected to the associated upstream and downstream supply tubes  21   a ,  21   b ,  21   c  or  21   d , respectively. The inlet pipe  41   a  and the outlet pipe  40   a , which communicate with the narrower groove, are connected to the associated upstream and downstream return tubes  22   a ,  22   b ,  22   c  or  22   d , respectively. 
     When each connector  40  and the associated connector  41  engage with each other, their wider grooves register or align with each other to form a supply passage  45 , and their narrower grooves register with each other to form a return passage  44 . As shown in FIG. 5, the ink from the inlet pipe  41   a  of the lower connector  41  flows from the adjacent end of the return passage  44  to the other end into the outlet pipe  40   a  of the upper connector  40 . Likewise, the ink from the inlet pipe  41   b  flows from the adjacent end of the supply passage  45  to the other end into the outlet pipe  40   b.    
     Each upper connector  40  has a protrusion  40   c  formed on its bottom. Each lower connector  41  has a recess  41   c  formed in its top for engagement with the protrusion  40   c . When the protrusion  40   c  of each connector  40  engages with the recess  41   c  of the associated connector  41 , the grooves in one of the connectors  40  and  41  register with the associated grooves in the other. A filter  42  is fitted in the recess  41   c  in such a manner that the filter covers the two grooves of the lower connector  41 . By engaging the protrusion  40   c  with the recess  41   c , it is possible to position the three members easily and securely. After the engagement, it is possible to seal the passages  44  and  45  securely by welding or bonding the connectors  40  and  41  by means of heat or ultrasonic waves, or joining them otherwise, with the filter  42  interposed between them. 
     The ink flowing through each upstream supply tube  21   a ,  21   b ,  21   c  or  21   d  and the associated downstream supply tube passes upward through the filter  42 . The ink flowing through each upstream return tube  22   a ,  22   b ,  22   c  or  22   d  and the associated downstream return tube passes upward through the same filter  42 . 
     The pipes  40   a  and  41   a  for the return passage  44  are positioned at both its ends. The pipes  40   b  and  41   b  for the supply passage  45  are positioned at both its ends. Therefore, when ink flows through the connectors  40  and  41 , it passes through relatively long areas of the filter  42  in the passages  44  and  45 . This increases the contact area between the filter  42  and ink. It is therefore possible to efficiently remove foreign substances etc. from the ink flowing through the filter  42 . 
     With reference to FIGS. 7-9, an ink jet recorder according to a third embodiment of the invention includes a head unit  2 , which includes a head holder  2   a . A manifold  51  is fixed to the front wall of the head holder  2   a  with a manifold holder  54 . A recording head  5   a  is mounted on the manifold  51 , and has an array of nozzles formed in it. An ink cartridge or tank  50  can be mounted on the head holder  2   a  to supply the head  5   a  with ink, and has an outlet  50   a  formed at its top. The manifold  51  has a bore  51   a  formed through it. The bore  51   a  includes an inlet port facing downward, a vertical part extending upward from this port, and a horizontal part extending forward from the top of the vertical part. The inlet port of the bore  51   a  is fitted with a seal member  52 , which may be made of rubber. The bore  51   a  can communicate with the cartridge outlet  50   a  through the seal member  52 . A filter  53  is interposed between the seal member  52  and the inlet port of the bore  51   a . The manifold  51  and the seal member  52  function as ink supply paths. 
     The head holder  2   a  includes a pawl or hook  2   b  formed at its bottom. The ink cartridge  50  includes a protrusion  50   b  formed on its rear wall. It is possible to mount the cartridge  50  on the head holder  2   a  by putting the cartridge in this holder, as shown in FIG. 8, in such a manner that the cartridge outlet  50   a  is positioned under the seal member  52  and the filter  53 , and by then engaging the cartridge protrusion  50   b  with the holder pawl  2   b , as shown in FIG.  9 . 
     This recorder includes a purging device (not shown), which is similar to the device  15  shown in FIG.  1 . Before the recorder starts recording, with the ink cartridge  50  mounted on the head holder  2   a , the purging device sucks ink from the recording head  5   a  through the nozzles. This causes ink to flow out of the cartridge  50  through the outlet  50   a , pass through the seal member  52 , the filter  53  and the manifold bore  51   a , and flow into the head  5   a . During the recording, the head  5   a  is replenished with ink from the cartridge  50  by the pressure drop due to the ejection of ink from the head. 
     When ink flows through the filter  53 , it moves upward. Therefore, air bubbles flowing with ink upward to the filter  53  pass through it easily with buoyancy, and are hardly or difficultly trapped by it. In particular, when the ink cartridge  50  is mounted, a large number of air bubbles are sandwiched between its outlet  50   a  and the filter  53 . However, the filter  53  traps few of these bubbles. 
     FIG. 10 shows the ink supply system for one of the recording heads  5   a - 5   d  (only  5   a  shown) of an ink jet color recorder according to a fourth embodiment of the invention. The heads  5   a - 5   d  are held by a head holder  2   c , which is the outer casing of the head unit  2 . The head holder  2   c  has four pairs of holes  2   b  formed through its bottom plate. The head unit  2  is mounted on a carriage  6 . 
     In particular, this recorder includes four pairs of joints  30  and  31  which are modifications of the joints  304  and  314  shown in FIG. 2 for the first embodiment. Each of the joints  30  and  31  is aligned with one of the holes  2   b  of the head holder  2   c . Each of the joints  30  is connected to a pair of supply tubes  21   a ,  21   b ,  21   c  or  21   d . Each of the joints  31  is connected to a pair of return tubes  22   a ,  22   b ,  22   c  or  22   d . The downstream supply tubes  21   a - 21   d  and the upstream return tubes  22   a - 22   d  are connected to the heads  5   a - 5   d , respectively. 
     With reference to FIGS. 11-13, each of the joints  30  includes an upper connector  30   b  and a seal member  30   a , which engage detachable with each other. The top of the connector  30   b  is connected to the associated downstream supply tube  21   a ,  21   b ,  21   c  or  21   d . The connector  30   b  has a conical bore  30   b - 1  formed near its bottom and widening downward. The seal member  30   a  is cylindrical and may be made of rubber or other elastic material. This member  30   a  has a cylindrical bore  30   a - 2  formed at its top. This member  30   a  also has a conical bore  30   a - 1  widening upward and positioned under the cylindrical bore  30   a - 2 . The bottom of the connector  30   b  engages detachable with the cylindrical bore  30   a - 2  of the seal member  30   a . This engagement causes the bottom of the conical bore  30   b - 1  of the connector  30   b  to face the top of the conical bore  30   a - 1  of the seal member  30   a . The sealing between the connector  30   b  and the seal member  30   a  is secured between the outer periphery of the bottom of the connector  30   b  and the inner periphery of the cylindrical bore  30   a - 2  of this member. 
     A filter  32  is supported at the bottom of the connector  30   b  and the top of the conical bore  30   a - 1  of the seal member  30   a  to remove foreign substances etc. from the ink flowing through the filter. The filter  32  may be fixed to one of the connector  30   b  and the seal member  30   a , or interposed between them. A passage is formed through the connector  30   b  and the seal member  30   a  in mutual engagement. The filter  32  is supported at the position in this passage where the diameter of the passage is largest. This enlarges the contact area between the filter  32  and ink. It is therefore possible to remove foreign substances etc. efficiently from the ink flowing through the filter  32 . 
     The seal member  30   a  has another conical bore  30   a - 3  formed at its bottom and widening downward. This member  30   a  has another cylindrical bore  30   a - 4  formed between the conical bores  30   a - 3  and  30   a - 1 . The bores  30   a - 1 ,  30   a - 2 ,  30   a - 3  and  30   a - 4  form a passage through the member  30   a.    
     The carriage  6  includes lower connectors  30   c  and  31   c  formed on and through it. These connectors  30   c  and  31   c  might otherwise be separate members fixed to the carriage  6 . Each of these connectors  30   c  and  31   c  is associated with one of the holes  2   b  of the head holder  2   c . Each of the connectors  30   c  extends through the associated hole  2   b  and the bottom conical bore  30   a - 3  of the associated seal member  30   a , and engages detachable in sealed condition with the lower cylindrical bore  30   a - 4  of this member. The bottom of the lower connector  30   c  is connected to the associated upstream supply tube  21   a ,  21   b ,  21   c  or  21   d.    
     Each of the other joints  31  includes an upper connector  31   b  and a seal member  31   a , which engage detachable with each other. The seal member  31   a  is identical with the seal member  30   a . The top of the upper connector  31   b  is connected to the associated upstream return tube  22   a ,  22   b ,  22   c  or  22   d . The upper connector  31   b  includes a conical bottom  31   b - 1  narrowing or converging downward for engagement with the upper conical bore  31   a - 1  in the seal member  31   a . This connector  31   b  also includes an outer cylindrical wall formed over the conical bottom  31   b - 1 . This wall engages detachable with the top cylindrical bore  31   a - 2  in the seal member  31   a . The sealing between the upper connector  31   b  and the seal member  31   a  is secured at one or both of the bores  31   a - 1  and  31   a - 2  in this member. 
     Similarly to the lower connector  30   c  for the joint  30 , each of the lower connectors  31   c  extends through the associated hole  2   b  of the head holder  2   c  and the bottom conical bore  31   a - 3  in the associated seal member  31   a , and engages detachable in sealed condition with the lower cylindrical bore  31   c - 4  in this member. The bottom of the lower connector  31   c  is connected to the associated downstream return tube  22   a ,  22   b ,  22   c  or  22   d . 
     The upper connector  31   b  has a cylindrical bore formed through it, which is substantially equal in diameter to the lower cylindrical bore  31   c - 4  in the seal member  31   a . Therefore, with this connector  31   b  in engagement with this member  31   a , no large step is formed between the connector bore and the lower cylindrical bore  31   c - 4 . Likewise, the lower connector  31   c  has a cylindrical bore formed through it, which is substantially equal in diameter to the lower cylindrical bore  31   c - 4  in the seal member  31   a . Therefore, a substantially straight passage is formed through the joint  31 . 
     The upper connectors  30   b  and  31   b  and the seal members  30   a  and  31   a  are fitted to the head holder  2   c  by a joint holder  38 . The joint holder  38  includes protrusions  38   b  formed inside it. The seal members  30   a  and  31   a  have outer peripheral grooves or recesses  30   a - 5  and  31   a - 5 , respectively, which engage with the protrusions  38   b  to hold these members in parallel. The joint holder  38  also includes pawls  38   c  formed at its top. The upper connectors  30   b  and  31   b  include inverted short skirts  30   b - 2  and  31   b - 2 , respectively. With these connectors  30   b  and  31   b  in engagement with the seal members  30   a  and  31   a , respectively, the skirts  30   b - 2  and  31   b - 2  engage with the pawls  38   c  to hold these connectors. The pawls  38   c  can elastically deform so that the upper connectors  30   b  and  31   b  can engage with the seal members  30   a  and  31   a.    
     The joint holder  38  includes hooks  38   a  provided around it. The head holder  2   c  includes pawls  2   a  provided on its bottom plate around every two holes  2   b . The joint holder  38  is mounted on the head holder  2   c , with each of the hooks  38   a  engaging with one of the pawls  2   a . The hooks  38   a  and the pawls  2   a  can elastically deform for mutual engagement. It is possible to mount the joint holder  38  on the head holder  2   c  by moving the joint holder downward until the hooks  38   a  engage with the pawls  2   a . The engagement of the hooks  38   a  with the pawls  2   a  limits the vertical movement of the joint holder  38 . 
     With the joint holder  38  mounted on the head holder  2   c , as shown in FIG. 11, there is a little play of “r”, between each of the hooks  38   a  and the associated pawl  2   a . The play of “r” allows the joint holder  38  to move a little horizontally on the head holder  2   c.    
     Back to FIG. 10, the head holder  2   c  includes pins  2   d  extending downward from its bottom plate. The carriage  6  has recesses  6   a  formed in its top plate for each engaging with one of the pins  2   d.    
     When the head unit  2  is mounted on the carriage  6 , the pins  2   d  engage with the recesses  6   a  to position the head unit  2  relative to the carriage  6 . This causes the eight lower connectors  30   c  and  31   c  to pass through the holes  2   b  of the head holder  2   c  and the bottom conical bores  30   a - 3  and  31   a - 3  of the eight seal members  30   a  and  31   a  into engagement with the lower cylindrical bores  30   a - 4  and  31   a - 4  of these members. The seal members  30   a  and  31   a  held by the four joint holders  38  can move horizontally within the range of the play of “r”. Therefore, even if there is a displacement or misalignment of “s” between each of the seal members  30   a  and  31   a  and the associated lower connector  30   c  or  31   c , it can be corrected or compensated. Because the bottom bores  30   a - 3  and  31   a - 3  of the seal members  30   a  and  31   a  are conical, the lower connectors  30   c  and  31   c  can be inserted easily in these members. The engagement of each of the joints  30  and  31  with the associated lower connector  30   c  or  31   c  connects the associated pair of upstream and down stream tubes  21   a ,  21   b ,  21   c ,  21   d ,  22   a ,  22   b ,  22   c  or  22   d . When the head unit  2  is removed from the carriage  6 , it is easy to disconnect each upstream tube and the associated downstream tube. 
     Thus, the joints  30  and  31  are positioned vertically in such a manner that ink can flow upward through the joints  30  for the supply tubes  21   a - 21   d  and downward through the joints  31  for the return tubes  22   a - 22   d . This allows air bubbles to pass buoyantly through the filters  32  in the joints  30  together with ink flowing upward. As stated above, no large step is formed between the bore in the upper connector  31   b  of each of the joints  31  and the lower cylindrical bore  31   a - 4  in the associated seal member  31   a . Therefore, few air bubbles will stay between these bores, and air bubbles can smoothly pass through the joints  31 . This always provides wide ink passages, resulting in good recording. 
     The seal members  30   a  and  31   a  are identical in shape, and can therefore be used in common. This reduces in number the parts forming the joints  30  and  31 , and therefore lowers the production costs for the joints. 
     The joint holders  38  hold the joints  30  and  31  with the play of “r” relative to the head holder  2   c . Therefore, even if each of the lower connectors  30   c  and  31   c  is not aligned slightly with the associated seal member  30   a  or  31   a  (displacement of “s”) when the head unit  2  is mounted on the carriage  6 , the associated joint  30  or  31  can be positioned within the range of the play of “r”. This makes it easy to connect each of the lower connectors  30   c  and  31   c  to the associated seal member  30   a  or  31   a  even if they are displaced slightly from each other when the head unit  2  is mounted on the carriage  6 . 
     With each of the upper connectors  30   b  in engagement with the associated seal member  30   a , the conical bore  30   b - 1  of the upper connector and the upper conical bore  30   a - 1  of the seal member widen toward the associated filter  32 . This allows ink to flow properly to the filter  32 , and therefore no special part is necessary for introducing ink to the filter  32 . Consequently, the parts forming each of the joints  30  are reduced in number, and it is therefore possible to simplify the process for assembling or engaging the parts of the joints  30 . It is also possible to downsize the joints  30 . 
     As will be apparent from the comparison between the prior art shown in FIGS. 28 and 29 and a tenth embodiment of the invention, which will be described later, there are few spaces or regions in each of the joints  30  where air bubbles tend to stay, because the bottom of the associated upper connector  30   b  is connected to the top of the top conical bore  30   a - 1  in the associated seal member  30   a . Therefore, few air bubbles impede the ink flow through the joint  30 . 
     FIGS. 14 and 15 show joints  30 ′ and  31 ′ according to a fifth embodiment of the invention. The joints  30 ′ for supply tubes  21   a ,  21   b ,  21   c  or  21   d  (only  21   a  shown) are identical with the joints  30  according to the fourth embodiment. Each of the joints  31 ′ for return tubes  22   a ,  22   b ,  22   c  or  22   d  (only  22   a  shown) includes an upper connector  31   b ′, which is identical with the upper connectors  30   b  of the joints  30 . 
     The joint  31 ′ also includes a seal member  31   a ′, which has a conical head  31   a ′- 4  narrowing upward for compressive contact with the side of the conical bore  31   b ′- 1  in the upper connector  31   b ′. The seal member  31   a ′ also has a cylindrical bore extending through the head  31   a ′- 4 . This member  31   a ′ further has a bottom conical bore  31   a - 3  formed under the cylindrical bore and widening downward. 
     The other parts of the joints  30 ′ and  31 ′ are substantially identical to the counterparts of the joints  30  and  31  according to the fourth embodiment. 
     There is no step where air bubbles tend to stay between the upper connector  31   b ′ and the seal member  31   a′.    
     FIG. 16 shows joints according to a sixth embodiment of the invention. These joints include a double connector  39 , which includes a pair of integral upper connectors. The connectors are connected by a bridge  39   a , and identical with the upper connectors  30   b  and  31   b  of the joints  30  and  31  according to the fourth embodiment. The other parts of these joints are substantially identical with the counterparts of the joints  30  and  31 . This may be applied likewise to the joints according to the fifth embodiment. The double connector  39  reduces the number of steps for fitting the joints to a joint holder  38 , and improves the productivity of the joints. 
     The foregoing embodiments may be modified in various ways. The conical bores in the seal members  30   a  and  31   a  and the upper connectors  30   b  and  31   b  widen outward, but might be shaped otherwise to guide ink smoothly. The filter  32  in each of the joints  30  might otherwise be fitted in the associated joint  31  for the associated return tubes  22   a ,  22   b ,  22   c  or  22   d  (only  22   a  shown). In this case, the joints  30  for the supply tubes  21   a - 21   d  (only  21   a  shown) might each have a straight passage formed through it. 
     FIGS. 17 and 18 show joints  30  and  310  according to a seventh embodiment of the invention. The joint  310  for a pair of return tubes  22   a ,  22   b ,  22   c  or  22   d  (only  22   a  shown) includes a seal member  310   a  and an upper connector  310   b , each of which has a cylindrical bore formed in it. The other parts of the joints  30  and  310  are substantially identical with the counterparts of the joints according to the fourth embodiment. It is not possible to use the seal members and the upper connectors in common to the joints  30  and  310 , as is the case with the fourth or fifth embodiment. The seventh embodiment is equivalent to the fourth and fifth embodiments, however, in that air bubbles can pass buoyantly through the filter  32  in the joint  30  together with ink flowing upward. This embodiment is also equivalent to those embodiments in that, because no large steps are formed between the upper connector  310   b  and the inner surfaces of the seal member  310   a  of the joint  310 , few air bubbles tend to stay there. 
     FIGS. 19 and 20 show joints  30  and  311  according to an eighth embodiment of the invention. The joints  30  and  311  include identical seal members  30   a . The joint  311  for a pair of return tubes  22   a ,  22   b ,  22   c  or  22   d  includes an upper connector  310   b , which has a cylindrical bore formed in it, as is the case with the sixth embodiment. The other parts of the joints  30  and  311  are substantially identical with the counterparts of the fourth embodiment. The seal members  30   a  can be used in common to the joints  30  and  311 . In the joint  311 , however, a large step is formed between the upper connector  310   b  and an inner surface of the seal member  30   a . In this respect, the fourth and fifth embodiments are preferable. 
     FIG. 21 shows the joint  300  for each pair of supply tubes  21   a ,  21   b ,  21   c  or  21   d , only the downstream tube  21   a  of which is shown, of an ink supply system according to a ninth embodiment of the invention. The joint  300  includes an upper connector  301 , a lower connector  302  and a seal member  303 . The upper connector  301  is connected to the associated downstream supply tube  21   a ,  21   b ,  21   c  or  21   d , and is fitted with a filter  32 . The upper connector  301  engages with the lower connector  302 , which engages with the seal member  303 . The upper connector  301  has a conical bore widening downward. The lower connector  302  has a conical bore widening upward. As is the case with the foregoing embodiments, the bottom of the seal member  303  engages with another connector (not shown), which is connected to the associated upstream supply tube. This upstream tube is connected to a sub tank. The lower connector  302  serves to introduce to the filter  32  the ink flowing from the tank. Air bubbles can pass buoyantly through the filter  32  together with ink flowing upward. In this respect, this embodiment and the foregoing embodiments are similar. 
     FIGS. 22 and 23 show a joint  30 ′ according to the tenth embodiment of the invention for solving the problem of the structure shown in FIGS. 28 and 29. The joint  30 ′ includes a double connector  39 , which includes integral twin connectors  30   b ′ and  31   b ′. The connectors  30   b ′ and  31   b ′ are connected to the downstream supply tube  21   a  and the upstream return tube  22   a , respectively. Each of the connectors  30   b ′ and  31   b ′ is fitted with a filter  32  on its bottom. The double connector  39  engages with a double seal member  140 , which includes integral twin sealers  30   a ′ and  31   a ′. The sealers  30   a ′ and  31   a ′ are connected directly to the upstream supply tube  21   a  and the downstream return tube  22   a , respectively. The seal member  140  is held by a holder  141 , which is mounted on a carriage (not shown). The double connector  39  also includes lock pawls  39   a . The holder  141  includes lock pawls  141   a  for each engaging with one of the pawls  39   a . Each of the twin connectors  30   b ′ and  31   b ′ includes a ridge  39   b  formed on its bottom. The ridge  39   b  engages with the top of the associated sealer  30   a ′ or  31   a ′ in such a manner that the sealer top deforms for effective sealing. 
     The double connector  39  and the double seal member  140  are connected end to end. This connection is held by the engagement of the lock pawls  39   a  and  141   a . With the connector  39  and the seal member  140  connected to each other, the top of the seal member deforms in accordance with the shape of the ridges  39   b  of the connector. This makes effective seals very near to the passages formed through the connector  39  and the seal member  140 , and to the filters  32  fitted to the connector. Therefore, few gaps are liable to be formed between each ridge  39   b  and the associated passages and filter  32 . Consequently, fewer air bubbles may stay in the passages. Because the seal member  140  is connected directly to the upstream supply tube  21   a  and the downstream return tube  22   a , the number of parts of the joint  30 ′ decreases. 
     FIGS. 24 and 25 show the joint  130  for each pair of supply tubes  21   a ,  21   b ,  21   c  or  21   d  (only  21   a  shown) of the ink supply system of an ink jet recorder according to an eleventh embodiment of the invention. The recorder includes a head holder  2   c ′, which can be mounted on a carriage  6 . 
     The head holder  2   c ′ includes cylindrical holders  2   a , standing on it. The joint  130  includes an upper connector  30   b  and a seal member  400 , which are fixed directly to one of the cylindrical holders  2   a′.    
     The carriage  6  includes cylindrical holders  6   b  protruding downward from it for each holding a lower connector  130   c . Each of these holders  6   b  includes an inner flange  6   b - 1  formed on it. The lower connector  130   c  includes an outer flange  130   d  formed on it. The cylindrical holder  6   b  surrounds the associated outer flange  130   d , which engages with the inner flange  6   b - 1  to keep the lower connector  130   c  from falling down. The inner flange  6   b - 1  can elastically deform so that the lower connector  130   c  can engage with and disengage from the cylindrical holder  6   b . The outer flange  130   d  engages with the cylindrical holder  6   b  with a little horizontal play of “r” around this flange. Thus, as shown in FIG. 25, the lower connector  130   c  is fitted to the carriage  6  with the horizontal play of  2   r.    
     When the head holder  2   c ′ is mounted on the carriage  6 , there may be a displacement of “s” between the seal member  400  and the lower connector  130   c . Even in this case, because the lower connector  130   c  can move horizontally within the range of  2   r  relative to the carriage  6 , it is possible to engage this connector and the seal member  400  accurately. Thus, because the position of the lower connector  130   c  can be varied within this range relative to the carriage  6 , it is easy to engage this connector and the seal member  400  when the head holder  2   c ′ is mounted on the carriage  6 . 
     In place of the lower connector  130   c  being movable relative to the carriage  6 , the upper connector  30   b  could move horizontally relative to the head holder  2   c′.    
     The structure of the cylindrical holder  6   b  on the carriage  6  and the outer flange  130   d  on the lower connector  130   c  might be applied to the head holder  2   c ′ and the upper connector  30   b , respectively. 
     The invention is not limited to the foregoing embodiments, which may be modified in various ways. 
     For example, each or one of the joint  30  or  130  for the supply tubes  21   a  and the joint  31  for the return tubes  22   a  might be fitted with a little play to the head holder  2   c  or the carriage  6 . 
     As shown in FIG. 2, the recording heads  5   a - 5   d  (only  5   a  shown) according to the first embodiment are so positioned as to eject ink horizontally. As shown in FIG. 26, however, the heads  5   a - 5   d  might be so positioned as to eject ink downward. With reference to FIG. 26, ink can flow upward through the filters  32  and  36 . This prevents the filters  32  and  36  effectively from trapping air bubbles. 
     In each of FIGS. 2 and 26, the filter  36  is fitted in the connector  35 , which engages with the manifold  27  of the recording head  5   a . Otherwise, the upstream return tube  22   a  might be bent properly to have a part where ink can flow upward. This tube part is fitted with a joint including a filter.