Ink jet recording apparatus

Abstract: The invention provides ink jet recording apparatus having a recording head (1) of the on demand type for ejecting ink droplets according to print data, and a platen (2) having a flat section (11c) opposed to the recording head (1) and arranged to extend at least over a region in which the printing head (1) is operable to effect printing. The flat section (11c) contains a plurality of openings (11a), and vacuum means (5) are provided to attract a recording medium (6) onto the flat section (11c) through the openings (11a) to enable the recording medium (6) to receive the ink droplets ejected from the head (1) for printing.

Description: The present invention relates to ink jet recording apparatus. A conventional ink jet recording apparatus is illustrated in Figures 5 and 6. As shown in Figures 5 and 6, conventionally a recording medium 17, such as recording paper sheet or film, wound around a platen 16 and is tensioned by a pair of pinch rollers 22a and 22b so as to achieve close contact between the recording medium 17 and the platen 16 to carry out printing. However, in the conventional recording apparatus, a distance H between a nozzle tip 20 of a recording head 18, which ejects ink droplets, and the recording medium 17 varies over an angular range of ±ϑ around a centre line of the platen 16. Further, in a multi-nozzle structure, the value of ϑ is relatively great, increasing the amount of variation of H, which causes degradation of print quality. In ink jet recording, it is necessary to equalise as much as possible the distances from the respective nozzle tips, arranged in multiples along a feeding direction (sub-scanning direction) of the recording medium, to the surface of the recording medium, and to minimise variation in the travelling time of the ink droplets in order to improve printing quality. When utilising a head of the on demand type, in order to cover the drawback of relatively low frequency in the ink droplet production, the multi-nozzle arrangement is normally adopted. However, as noted above, it is difficult to prevent degradation of printing quality without enlarging the platen diameter. Another known ink jet recording apparatus can be found in DE-A-32 35 593 which discloses a platen which is a perforated hollow cylinder subjected to a low pressure, the surface of which being provided with openings communicating with the inside of the cylinder and channels co-operating with said openings in order to maintain a recording medium properly on the platen. It is an object of the present invention to overcome the above problems. According to the present invention, there is provided an ink jet recording apparatus, comprising: a recording head for ejecting ink droplets, in response to print data; a platen for supporting a recording medium, said platen having a plurality of openings arranged to cooperate with means for generating a reduced pressure therein, to attract a recording medium, characterised in that said platen has a flat section which is opposed to the recording head and extends over a region in which the recording head is operable to effect printing, and said openings are arranged to attract a recording medium onto said flat section. The recording apparatus according to the invention described below comprises a platen having a flat section whose extent corresponds to a printing width in which a recording head is operable to effect printing, and vacuum means for attracting a recording medium. After placing the recording medium between the head and the platen, the vacuum means is operated to attract the recording medium onto the flat section of the platen so as to control the distance between the head and the recording medium to be constant, whereby high accuracy in ink droplet targeting and improved print quality may be achieved. The flat section of the platen is preferably formed with a plurality of generally circular openings having a relatively small diameter and formed to co-operate with the vacuum means to attract the recording medium onto the flat section so as to control and maintain the positioning of the recording medium and thereby avoid floating of the recording medium. In use of such apparatus, a recording medium, for example paper or film, is inserted into the ink jet recording apparatus to carry out printing. The recording medium is gradually attracted onto the flat section of the platen within a region corresponding to a printing width or span of the recording head by means of the attractive force generated in the small diameter openings in the platen face due to the flow of air caused by the vacuum means. During the course of the attraction of the recording medium, at least a substantial number of the openings are closed so as to reduce the amount of air flow generated by the vacuum means. Consequently, air pressure is abruptly reduced between the recording medium and the platen to thereby boost the attractive force. With increase of the attractive force, the recording medium is completely attracted onto the platen so that its surface achieves a flatness identical with that of the platen flat section and floating is avoided. Depending on the size of the recording medium in the width-wise direction (main scanning direction), a number of the openings may be off-set from the span of the recording medium causing a reduction of the attractive force. For example, in the case of a recording medium of A4 size loaded into an apparatus which can print up to an A0 size of recording medium, three quarters of the area in which the openings are provided is out of the span of the A4 size recording medium. In order to avoid a substantial reduction in the attractive force, the dimension or density of the openings is gradually reduced in the width-wise direction away from a guide for width-wise positioning of the recording medium, so as to compensate for differences in the width of recording medium and thereby effectively avoid floating thereof. The invention is described further, by way of example, with reference to the accompanying drawings, in which:- Figure 1 is a sectional view showing one embodiment of ink jet recording apparatus according to the present invention; Figure 2 is a perspective view of the recording apparatus; Figure 3 is a plan view of a platen of the recording apparatus; Figure 4 is a plan view of an alternative platen; Figure 5 is a sectional view of a conventional ink jet recording apparatus; and Figure 6 is a plan view of the conventional recording apparatus. An embodiment of the invention will be described with reference initially to Figures 1 and 2, which show ink jet recording apparatus including a platen 2. The platen 2 has a plurality of generally circular openings 11a having relatively small diameters, and a plurality of window openings 11b for receiving driving rollers 3 which feed a recording medium 6. A front face of the platen has a flat section 11c extending over a region corresponding to a printing span of a recording head 1 comprising a multi-nozzle ink jet recording head of the on demand type. A vacuum space 11d is formed inside the platen. The vacuum space 11d communicates with the flat section 11c on the front face of the platen 2 through the plural openings 11a whereby to attract the recording medium 6 in use. The width of the vacuum space 11d and the width of the arrangement of the openings 11a are set to be identical to the maximum width of recording medium which can be printed by the recording apparatus. In this embodiment, the width is set to be about 841 mm for A0 size. As shown in Figure 3 or Figure 4, the openings 11a are arranged such that a total opening area A of the openings 11a per unit surface area W is gradually reduced away from a guide 11e, which determines the width-wise position of the recording medium. In the Figure 3 embodiment, the openings 11a each have a 3 mm diameter and are arranged at a pitch L₁ = 20 mm within a width-wise span corresponding to an A3 size of recording medium. Then, the openings 11a are arranged at a pitch L₂ = 30 mm beyond the A3 size span and within an A2 size span. Thereafter, the openings 11a are arranged at a pitch L₃ = 40 mm beyond the A2 size span and within an A0 size span. In the Figure 4 embodiment, all of the openings 11a are arranged at a pitch of L₁ = L₂ = L₃ = 20 mm, and the diameter of the openings 11a is gradually changed in correspondence with the width-wise span of different recording medium. In particular, the diameter ⊘ is set to be 3 mm within a span corresponding to an A3 size of recording medium, and is set to be 2.35 mm beyond the A3 size span and within the A2 size span, and further is set to be 1.86 mm beyond the A2 size span and within the A0 size span. On the other hand, the openings 11a are arranged in the feeding direction (sub-scanning direction) so as to cover substantially an area opposed to and in the range of a multi-nozzle face 7b of the head 1. In this instance, the total length of the multi-nozzle arrangement is set to be 8 mm since 64 nozzles are arranged vertically at a pitch of 1/8 mm. Thus, in both embodiments, the openings 11a are arranged in the sub-scanning direction at a common pitch of 20 mm in two sets so as to cover a range from a minimum 20 mm to a maximum 40 mm. Vacuum means in the form of a fan 5 suck air from within the vacuum space 11d inside the platen 2. The driving rollers 3 with pinch rollers 8 sandwich the recording medium 6 therebetween to feed the same in the sub-scanning direction. A carriage 13 carries the head 1, and is supported by guide shafts 14 and is driven in a direction (main scanning direction) transverse to the feeding direction by means of a head-feeding servo motor etc. (not shown) through a wire or belt and a spool (also not shown) so as to undergo reciprocating movement. The recording medium 6 is fed from a rear part to a front part of the recording apparatus by means of the driving rollers 3 and the pinch rollers 8. The driving rollers 3 receive a drive force from a pulse motor (not shown) through a timing belt and a spool. An ink supply tube 12 supplies ink to the head 1 from an ink cartridge (not shown). A flexible circuit substrate 15 applies to each nozzle of the head 1 a driving pulse based on printing data. As described before, the head 1 comprises a multi-nozzle ink jet recording head of the on demand type. A number 64 of the nozzles are linearly arranged in the sub-scanning direction at a pitch of eight nozzles per millimetre and operate to eject ink droplets towards the recording medium on a demand basis in response to drive pulses input from the flexible circuit substrate 15 according to printing data. Next, a description will be given of a scanning operation of the above constructed embodiment of the recording apparatus. The recording medium 6 is set in the initial position such that a front edge thereof is sandwiched between the driving rollers 3 and the pinch rollers 8 in the sub-scanning direction, and a rear part thereof passes between the head 1 and the flat section 11c of the platen 2, which defines a guide face for the recording medium 6. The recording medium 6 extends rearwardly of the apparatus in a free state. Then, the fan 5 is operated to initiate suction. By this suction, air is withdrawn from the vacuum space 11d inside the platen 2 and is evacuated outside the platen 2 underneath the fan 5. Consequently, the air pressure in the space 11d is reduced such that air flows from the outside of the platen 2 along the recording medium 6 into the space 11d through the openings 11a so as to generate an attractive force. Due to this attractive force generated in the openings 11a, the recording medium 6 begins to be brought into close contact with the flat section 11c of the platen 2. By this contact, many of the openings 11a are closed to reduce the amount of air flowing from the openings 11a to the inside of the platen 2. Consequently, the air pressure is abruptly reduced in a gap between the recording medium 6 and the platen 2 to thereby boost the attractive force. With an increase in the attractive force, the recording medium 6 is made to contact closely the flat section 11c of the platen 2 such that its flatness becomes identical to that of the flat section 11c and floating of the recording medium 6 is removed. The flat section 11c of the platen 2 is precisely finished to achieve a flatness with an accuracy of less than 0.2 mm over an entire area within the printing span of the head 1, and moreover it has a sufficient stiffness effective to avoid deformation, such as bending, due to the drawing of the fan. Generally, in ink jet printing, it is necessary to maintain the distance between the head 1 and the platen 2 to an amount in the order of 1.0 mm to 1.2 mm. According to the present invention, the recording medium can be stably placed within an allowance comparable to the variation of the flatness of the platen 2. The operation following completion of the attraction of the recording medium 6 is as follows: While scanning the carriage 13 in the main scanning direction in response to a printing start signal, the recording head 1 mounted on the carriage is operated to eject ink droplets to effect printing according to print data. A printed pattern and an ink jet amount. can be determined according to the print data supplied through the flexible circuit substrate or cable. The printing interval in the sub-scanning direction is determined by the total number of nozzles arranged on the head 1 and the pitch thereof. In this embodiment, the printing interval is set to be 8 mm as described before. Accordingly, with each line scanning of the carriage 13 in the main scanning direction, the recording medium 6 is intermittently fed forwardly relative to the recording apparatus by an 8 mm step through the driving rollers 3 and the pinch rollers 8. At this time, the attractive force applied to the recording medium 6 by way of the openings 11a can produce an appropriate back tension for feeding of the recording medium so as to facilitate stable feeding. During the course of the printing operation in a manner as described above, the recording medium 6 can be continuously attracted through the edge thereof to avoid any drawback such as floating. A description is now given for printing different widths of recording medium 6 in the present recording apparatus. The attractive force applied to the recording medium 6 is boosted by closing the openings 11a with the recording medium 6 so as efficiently to avoid floating. Therefore, when inserting a recording medium 6 having a relatively small print width, such as A4 and A3 size, into the recording apparatus, which has a relatively large maximum printing span covering an A0 size of 841 mm in this embodiment, one half to three quarters of the openings are off-set from the recording medium 6, whereby a strong attractive force may fail to be generated. In the present invention, in order to compensate for such potential drawback, the openings 11a are arranged such that the total opening area thereof per unit surface area of the platen is gradually decreased in the width-wise direction of the recording medium so as to avoid considerable reduction of the attractive force even when a portion of the plural openings is off-set from the recording medium 6 when it has a small size. In particular, the pitch of the openings is increased in correspondence with the width of the recording medium 6, while the diameter of the openings is reduced accordingly to effect the compensation. Figures 3 and 4 show examples of such arrangement and structure. In this instance, the total opening area is gradually changed per unit surface area W = 2500 mm², such that the total opening area in the unit surface is set to be about 92 mm² within the span of A3 size, then to be about 56.5 mm² within the span of A2 size, and further to be about 35 mm² within the span of A0 size. According to experimental results when the openings are arranged as described above, the attractive force applied to a smaller size of recording medium 6 is reduced only by about 20% as compared to a larger size of recording medium for which all of the openings 11a are closed, thereby efficiently ensuring an effective attraction force applied to the smaller size of recording medium relative to the face of the platen 2 to avoid floating. Consequently, good printing can be carried out for various sizes of the recording medium 6 without degradation of print quality. As described above, according to the present invention, a recording medium can be closely contacted onto a flat section of platen within a region corresponding to the printing span of a head through an attractive force produced by vacuum means and openings having a relatively small diameter so as to avoid floating. Thereby the distance between the nozzle tips of the recording head and the recording medium may be controlled and maintained constant over the entire multi-nozzle arrangement. Consequently, an ink jet recording apparatus can be provided such that a reduction in targeting accuracy of the ink droplets can be avoided to obtain a high quality of print image with highly accurate dot positioning.

Claims: Ink jet recording apparatus, comprising: a recording head (1) for ejecting ink droplets, in response to print data; a platen (2) for supporting a recording medium (6), said platen (2) having a plurality of openings (11a) arranged to cooperate with means (5) for generating a reduced pressure therein, to attract a recording medium, characterised in that said platen (2) has a flat section (11c) which is opposed to the recording head and extends over a region in which the recording head is operable to effect printing, and said openings are arranged to attract a recording medium onto said flat section. Ink jet recording apparatus according to claim 1 characterised in that the openings are arranged such that a total opening area of the openings per unit surface area of the flat section varies in a width-wise direction. Ink jet recording apparatus according to claim 2 characterised in that the total opening area of the openings per unit surface area is reduced in the width-wise direction. Ink jet recording apparatus according to claim 2 or claim 3 characterised in that the total opening area is varied by changing the pitch between adjacent openings in the width-wise direction. Ink jet recording apparatus according to claim 2 or 3 characterised in that the openings are generally circular and in that the total opening area is varied by changing the diameter of the openings in the width-wise direction. Ink jet recording apparatus according to claim 5 characterised in that the openings have a relatively small diameter. Ink jet recording apparatus according to any of claim 2 to 6 characterised in that the total opening area of the openings per unit surface area of the flat surface is varied successively. Ink jet recording apparatus according to any preceding claim, wherein the recording head (1) is an on demand type recording head.

Inventors: DENDA MASAKI C O SEIKO INSTRUM, DENDA, MASAKI, C/O SEIKO INSTRUMENTS INC.
Applicants: SEIKO INSTR INC, SEIKO INSTRUMENTS INC.
Application year: 1990
Main classification: B41J3
IPC codes: B41J11, G09F7, G09F13, G09F19, B41J15, B41J2