JP2003039753A - Inkjet image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To surely prevent ink from being adhered to a rear face of a recording paper even when ink is ejected on a platen and to remove the ink from the platen without executing a specific removing operation. SOLUTION: A plurality of ribs 71, 72 extending in a conveyance direction of a recording paper are provided to be projected on the surface of the platen 7. Ink introducing grooves 75, 76 extending in the conveyance direction of the recording paper are formed at the central section of the ribs 71, 72 in the width direction. Each of the ink introducing grooves 75, 76 is formed such that the cross section thereof is reduced gradually as it goes toward the downstream side in the conveyance direction of the recording paper. The ink in the ink introducing grooves 75, 76 is allowed to flow toward a portion having a smaller cross section by a capillarity to be discharged from the platen 7.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an inkjet image forming apparatus for forming an image on a recording sheet (recording medium) by ejecting ink droplets. In particular, the present invention relates to an improvement of a platen that contacts the back surface of a recording sheet and guides the recording sheet.

[0002]

2. Description of the Related Art Generally, in an ink jet type image forming apparatus (hereinafter referred to as an ink jet printer), ink droplets are ejected onto the upper surface of a continuously fed recording sheet to sequentially form an image. As a method of this image formation,
A method is generally known in which a print head and an ink tank are mounted on a carriage, and the carriage reciprocates in the main scanning direction orthogonal to the conveyance direction of the recording paper to eject ink droplets from the print head onto the upper surface of the recording paper. There is.
Further, a platen is disposed so as to face the print head, and the conveyed recording paper is subjected to the image formation while being guided on the platen.

By the way, in the ink jet printer,
When the malfunction causes the ink droplets to be ejected from the print head in the absence of the recording paper on the platen or the ink droplets to be ejected to the area off the recording paper conveyed on the platen, or If ink leaks from the print head without receiving the image formation command signal, the ink adheres to the upper surface of the platen. In this state, when the recording paper is conveyed to the ink-adhering portion on the platen, the ink on the platen adheres to the back surface of the recording paper and stains the recording paper.

As a means for solving this inconvenience, hitherto, JP-A-7-9712 and JP-A-7-28525 have been proposed.
An inkjet printer disclosed in Japanese Patent Publication No. 1 has been proposed.

In the former publication, the platen itself is formed of a material having an ink absorbing property, or a notch is formed on the surface of the platen and an ink absorbing material made of a material having an ink absorbing property is formed in the notch. A technique for providing a member is disclosed. This prevents the ink ejected onto the platen from being collected by the platen itself or the ink absorbing member and adhered to the back surface of the paper.

On the other hand, in the latter publication, by providing the platen with an inclined surface inclined obliquely upward toward the downstream side in the sheet conveying direction, the recording sheet can be guided in a scooping manner and the leading edge of the sheet can be surely guided. It was done like this,
A technique of forming an ink reservoir groove on this inclined surface is disclosed. As a result, when the ink is ejected onto the platen, the ink is poured into and held in the ink reservoir groove. This makes it possible to prevent the ink on the platen from adhering to the back surface of the paper.

[0007]

However, the techniques disclosed in the above-mentioned publications have the following problems.

First, in the one disclosed in JP-A-7-9712, there is a limit to the amount of ink that can be absorbed by the platen itself and the ink absorbing member. Therefore, if the amount of ink ejected onto the upper surface of the platen is relatively large, it may not be possible to absorb all the ink.
That is, the ink that cannot be absorbed by the platen itself or the ink absorbing member adheres to the back surface of the recording paper and stains the recording paper. Further, in order to avoid such a situation, it is conceivable to replace the ink absorber regularly, but this will compel the user to perform a complicated replacement work.

Similarly, even in the one disclosed in Japanese Patent Laid-Open No. 7-285251, in order to prevent the ink held in the ink reservoir groove from adhering to the back surface of the recording paper, this This is impractical because it requires a mechanism and work for removing the ink held in the ink reservoir groove.

The present invention has been made in view of the above points, and an object thereof is to adhere the ink to the back surface of the recording paper even if the ink is ejected onto the platen. That is, it is possible to surely prevent the ink from being ejected, and to remove the ink from the platen without requiring a special removing operation.

[0011]

Means for Solving the Problems-Outline of the Invention-In order to achieve the above-mentioned object, the present invention has ribs projecting from the upper surface of a platen and a recording sheet (recording medium) supported by the tips of the ribs. By transporting the ink while being conveyed, it is possible to avoid contact between the ink ejected on the platen and the recording paper. Further, means for discharging the ink from the platen by the capillary phenomenon is provided.

-Solution Means-Specifically, a platen for guiding the recording medium and an image forming means for ejecting ink droplets to the recording medium which is arranged facing the platen and is conveyed along the platen are provided. It is premised on an inkjet image forming apparatus. In this inkjet image forming apparatus, a plurality of ribs extending in the recording medium conveyance direction are provided on the surface of the platen so as to project. On the other hand, an ink lead-out groove that is recessed in the ink ejection direction and extends in the recording medium conveyance direction is formed in each rib. The cross-sectional area of the ink lead-out groove is formed so as to gradually decrease toward at least one of the upstream side and the downstream side in the recording medium conveyance direction.

Due to this specific matter, when the ink ejected from the image forming means adheres to the platen due to a malfunction of the image forming apparatus, the ink is led out between the ribs on the platen or the ink formed on the ribs. It flows into the groove. Since the recording medium is supported and conveyed by the tips of the ribs, the ink on the platen does not come into contact with the recording sheet, and the back surface of the recording sheet can be prevented from being contaminated with the ink. In addition, the ink that has flowed into the ink lead-out groove flows toward the direction in which the cross-sectional area of the ink lead-out groove becomes smaller due to the "capillary phenomenon", that is, toward the upstream side or the downstream side in the recording medium conveyance direction. For this reason,
The ink that has flowed into the ink lead-out groove does not remain in the ink lead-out groove and is discharged to the outside of the platen. Therefore, even when the amount of ink ejected onto the platen is relatively large, the back surface of the recording paper is not contaminated by this ink.

The following are specific examples of the shape of the ink lead-out groove. First, the ink lead-out groove is formed so that the groove width dimension of the open end in the cross section orthogonal to the recording medium conveyance direction gradually decreases toward at least one of the upstream side and the downstream side in the recording medium conveyance direction.

Further, the ink lead-out groove is formed so that the groove width dimension orthogonal to the recording medium conveyance direction gradually decreases toward the inner side in the depth direction.

According to the former structure, it is possible to easily realize a structure for surely moving the ink accumulated in the ink lead-out groove in the direction in which the groove width dimension decreases due to the capillary phenomenon.
On the other hand, according to the latter configuration, the ink introduced into the ink lead-out groove tends to proceed to the bottom of the ink lead-out groove due to the capillary phenomenon in addition to the gravity. Therefore, it is possible to reliably prevent the ink from overflowing the ink outlet groove and adhering to the back surface of the recording medium.

If an ink recovery member for recovering the ink led out from the ink lead-out groove is provided near the end of the ink lead-out groove where the cross-sectional area is the smallest, the ink lead-out groove is moved. The generated ink can be recovered by the ink recovery member, and it is possible to prevent the ink discharged from the ink lead-out groove from returning to the platen again.

When the inner surface of the ink lead-out groove is formed of a material having hydrophilicity or lipophilicity, the adhesive force between the ink and the inner surface of the ink lead-out groove in the ink lead-out groove increases. Therefore, it is possible to further enhance the effect of the capillary phenomenon and accelerate the discharge of the ink from the ink lead-out groove.

When the outer surface of the rib is made of a material having water repellency or oil repellency, the ink ejected on the side wall of the rib easily flows down toward the lower part of the rib, and the ink is stagnant at the upper end of the rib. Then, it is possible to reliably avoid the situation where the toner adheres to the recording medium.

Further, as compared with the case where a recording medium having an extension portion extending from the end portion in the direction orthogonal to the transport direction to the platen surface side is adopted, the outer surface shape of the rib is the same as the end portion of the recording medium. The distance between the portion and the outer surface of the rib is set to be equal to or longer than the length dimension in the extension direction of the extension portion. For example, if an envelope is used as the recording medium,
It is possible to prevent the tip edge of the lid portion (extension portion) from being pressed by the rib. For this reason, it becomes possible to favorably convey the lid portion without causing any bending.

[0021]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings. In this embodiment, a case will be described where the present invention is applied to a color inkjet printer having a serial head as an image forming apparatus.

-Description of Overall Structure of Inkjet Printer- Fig. 1 shows a color inkjet printer 1 according to the present embodiment.
It is a figure which shows the internal structure of. As shown in FIG. 1, the inkjet printer 1 according to this embodiment includes a paper feed unit 2, a separation unit 3, a conveyance unit 4, an image forming unit 5, and a discharge unit 6.

The paper feed unit 2 is provided with a paper feed tray 21 extending in a substantially vertical direction and a pickup roller (not shown). When the printing is started, the recording paper P as a recording medium in the paper feed tray 21 is taken out by the pickup roller. It is adapted to be conveyed toward the separating section 3. Further, the paper feed tray 21 is provided with the recording paper P when printing is not performed.
Function as a storage unit of.

The separation unit 3 is for supplying the recording paper P supplied from the paper supply unit 2 to the image forming unit 5 one by one, and is provided with a paper supply roller 31 and a separator 32. There is. In the separator 32, even if a plurality of recording sheets P, P, ... Are taken out by the pickup roller, the plurality of recording sheets P, P ,. Only the parts can be sent to the transport section 4.

The transport section 4 is for transporting the recording sheets P supplied one by one from the separation section 3 toward the image forming section 5, and is provided with a guide plate 41 and a pair of transport rollers 42. . The transport roller pair 42 is a member that adjusts the transport of the recording sheet P so that the image is formed at an appropriate position when the recording sheet P is sent to the image forming unit 5.

The image forming section 5 is for performing image formation (printing) on the recording paper P supplied from the pair of conveying rollers 42 of the conveying section 4, and is a recording head 52 as an image forming means, and this recording head. A carriage 51 equipped with 52,
A guide shaft 53 for guiding the carriage 51 in the main scanning direction and a platen 7 functioning as a support for the recording paper P during printing are provided. When the ink droplets are ejected from the recording head 52 toward the recording sheet P, the platen 7 abuts and supports the back surface of the recording sheet P to prevent the recording sheet P from bending. ing. The detailed configuration of the platen 7 will be described later.

The discharging section 6 is a section for collecting the recording paper P on which an image has been formed, an ink drying section (not shown) for drying the ink on the recording paper P, and a discharging roller 61.
And a discharge tray 62.

In the above structure, the ink jet printer 1 prints by the following operation. First,
A print request based on image information is issued to the inkjet printer 1 from a computer or the like (not shown). Upon receiving the print request, the inkjet printer 1 carries out the recording paper P on the paper feed tray 21 from the paper feed unit 2 by the pickup roller. Next, the recording paper P that has been carried out
Is passed through the separation unit 3 by the paper feed roller 31 and sent to the transport unit 4. In the transport unit 4, the recording sheet P is sent to the image forming unit 5 by the pair of transport rollers 42. Then, in the image forming section 5, the image forming operation on the recording paper P is performed by a predetermined image forming operation. The recording sheet P on which the image has been formed is discharged to a discharge tray 62 by a discharge roller 61 via an ink drying unit (not shown). As a result, the recording paper P is provided to the user as a printed matter.

-Description of Platen 7 Structure-Next, the structure of the platen 7, which is a characteristic member of the present embodiment, will be described. FIG. 2 is a plan view of the platen 7 according to this embodiment (viewed from the recording head 52 side). In this figure, the recording paper P is conveyed from the upper side to the lower side in the figure as indicated by the arrow C in the figure.

The platen 7 is formed of a substantially rectangular plate-like body in which the side orthogonal to the side extending in the recording sheet conveyance direction is set longer. Ribs 71, 71, ..., 72, 7 are provided at a plurality of locations on the upper surface of the platen 7.
2, ... Are integrally formed. Hereinafter, these ribs 71,
The shapes of 71, ..., 72, 72 ,. The ribs 71, 71, ..., 72, 72, ... Extend along the recording paper conveyance direction (vertical direction in FIG. 2), and in the present embodiment, in the main scanning direction (direction orthogonal to the paper conveyance direction). It is formed at 9 locations with equal intervals. Of these nine ribs 71, 71, ..., 72, 72, ..., the five ribs on the left side in the figure are the first ribs 71, 71, ..., and the four ribs on the right side in the figure are the second ribs 72, 72. , ... First rib 71 and second
The ribs 72 are different from each other in shape. Details of these shapes will be described later.

An ink collecting member 8 made of an absorptive material such as porous ceramics is arranged at the edge of the platen 7 on the downstream side in the sheet conveying direction. The ink collecting member 8 collects the ink that has adhered to the platen 7 after the ink has been drawn out by the drawing operation described later.

The shapes of the ribs 71 and 72 will be described below. FIG. 3 shows a sectional shape of the first rib 71.
3A is a sectional view taken along the line AA in FIG. 2, and FIG. 3B is a sectional view taken along the line BB in FIG.
FIG. 3C is a sectional view taken along the line C-C in FIG. 2.
Further, FIG. 4 shows a sectional shape of the second rib 72. 4A is a cross-sectional view taken along the line DD in FIG.
4B is a sectional view taken along line EE in FIG.
FIG. 3C is a sectional view taken along the line FF in FIG.

As shown in these figures, each rib 71, 7
, 72, 72, ... are directions from the surface of the platen 7 toward the recording head 52 (upper side in FIGS. 3 and 4).
Is formed so as to project. In addition, each rib 71, 71,
, 72, 72, ... Ink leading grooves 75, 76 having a predetermined shape are formed in the central portion of the upper surface. In FIG. 2, the ink lead-out grooves 75 and 76 are formed with diagonal lines.

As the shapes of the ink lead-out grooves 75 and 76, the depth dimension is set to be constant over the entire sheet conveying direction (the vertical direction of the paper surface of FIGS. 3 and 4). The groove widths of the ink lead-out grooves 75 and 76 are formed so as to become gradually narrower in the direction in which the ink is ejected (the depth side of the groove). Furthermore, the ink lead-out grooves 75, 76
Are formed such that the groove width at the upper end gradually decreases toward the downstream side in the paper transport direction (dimensions A1, A2, A3 in FIG. 3 and dimensions B1, B2 in FIG. 4).
See B3). In this way, the ink lead-out grooves 75 and 76 of the ribs 71 and 72 have a substantially inverted trapezoidal cross section, the trapezoidal height is constant, and the dimensions of the upper and lower bottoms are the recording paper conveyance direction. The shape is such that it becomes gradually shorter toward the downstream side. That is, the cross-sectional areas of the ink lead-out grooves 75 and 76 are formed so as to gradually decrease toward the downstream side in the recording paper conveyance direction.
In addition, the cross-sectional area of the ink lead-out grooves 75 and 76 referred to here is
This is the area of a region surrounded by the peripheries of the ink lead-out grooves 75 and 76 and the line segment connecting the upper ends of the ink lead-out grooves 75 and 76 in the cut surfaces of the ribs 71 and 72 in the direction orthogonal to the recording paper conveyance direction.

Further, the outer surface 7 of the side wall 73 of the first rib 71
3a is made of a material having water repellency or oil repellency, such as a fluorine-based resin. Similarly, the outer surface 74a of the side wall 74 of the second rib 72 is also made of a water-repellent or oil-repellent material such as a fluorine resin.

On the other hand, the inner surface 75a of the ink lead-out groove 75 of the first rib 71 is made of a material having hydrophilicity or lipophilicity such as Parisden. Similarly, the inner surface 76a of the ink lead-out groove 76 of the second rib 72 is also formed of a material having hydrophilicity or lipophilicity such as Parisden.

-Description of Function of Platen 7- Next, the function of the platen 7 configured as described above will be described.

During the image forming operation on the recording paper P,
When the ink ejected from the recording head 52 is ejected to a region outside the recording paper P and adheres to the upper surface of the platen 7, the platen 7 performs the ink discharging operation as follows.

In other words, the ink attached to the upper surface of the platen 7 has the ribs 7 on the upper surface of the platen 7.
1, 71, 72, a region between the 72 or ribs 71,
It flows into the ink lead-out grooves 75 and 76 of 72. In particular, since the outer surfaces 73a and 74a of the side walls 73 and 74 of the ribs 71 and 72 are formed of a material having water repellency or oil repellency, the ink ejected onto the ribs 71 and 72 can smoothly flow through the ribs 71 and 72. Run down to the bottom of 72. That is,
The ink does not stay on the upper edges of the ribs 71 and 72. That is, the recording paper P has ribs 7 in which ink is present.
1, 71, 72, areas between the 72 or ribs 71, 7
Since it is conveyed while being supported by the platen 7 at a position above the second ink lead-out grooves 75 and 76,
This ink does not adhere to the back surface of the recording paper P and stain the recording paper P.

Then, the ink on the platen 7 is discharged from the platen 7 by the following operation. That is, as described above, the ink lead-out groove 7 of each of the ribs 71, 72.
Since the inner surfaces 75a, 76a of the ink tanks 5, 76 are formed of a material having hydrophilicity or lipophilicity, the ink lead-out grooves 75,
The adhesive force by which the ink flowing into 76 wets the wall surfaces of the ink outlet grooves 75, 76 is larger than the cohesive force between the ink molecules. Therefore, the ink ejected directly from the recording head 52 to the ink lead-out grooves 75 and 76 is
The so-called "capillary phenomenon" progresses in the direction of the smaller cross-sectional area. Therefore, the ink that has flowed into the ink lead-out grooves 75 and 76 does not remain in the ink lead-out grooves 75 and 76 but travels outside the platen 7, and the ink collecting member 8
Will be collected by. As a result, even if the ink is accidentally ejected onto the platen 7, the ink is discharged to the outside of the platen 7 while preventing the ink from adhering to the back surface of the recording paper P. You can

Next, the difference between the first rib 71 and the second rib 72 will be described. As shown in FIG. 3, the first rib 71
The outer surfaces 73a, 73a of the side walls 73, 73 are slightly curved and symmetrical. On the other hand, as shown in FIG. 4, the outer surface 7 of each side wall 74, 74 of the second rib 72 is
The shapes of 4a and 74a are asymmetrical.

More specifically, the shape of the outer surface 74a of the right side wall 74 in FIG. 4 is slightly curved like the first rib 71. On the other hand, the left side wall 7 in FIG.
The outer surface 74a of No. 4 is formed by a flat surface (tapered surface) that inclines toward the center side of the second rib 72 as it goes upward. As shown in FIG. 5A (this figure shows a state where the upper end portion of the envelope P1 is conveyed when printing is performed on the envelope P1), the flat surface P1
The distance between the end of the envelope and the upper end of this flat surface (dimension T1 in FIG. 5A) is the width of the lid portion (extension in the present invention) P2 of the envelope P1 (dimension T2 in FIG. 5A). It is formed as described above. Hereinafter, the shape of this flat surface will be described in detail.

As described above, FIG. 5 shows a case where the front face of the envelope P1 is printed by the ink jet printer 1 and the envelope P1 is being conveyed on the platen 7. The position of the second rib 72, which is the third rib from the right in FIG. 2, is set so as to correspond to the lid portion P2 of the envelope P1. FIG. 2 shows the size of the envelope P1 and the size of A4 paper on the platen 7, respectively. 5A shows the case of this embodiment in which the third rib from the right is formed as the second rib 72, and FIG. 5B shows the third rib from the right as a comparative example. The case where it is formed as the first rib 71 is shown.

As shown in FIG. 5 (b), when the third rib from the right is the first rib 71, it is between the end of the envelope P1 and the outer surface 73a of the side wall 73 of the first rib 71. Distance T
3 becomes shorter than the width of the lid portion P2 (dimension T2 in FIG. 5a), and a force may be generated that presses the lid portion P2 against the side wall 73 of the first rib 71 when the envelope P1 is conveyed. Bending occurs in the portion P2. In such a case, conveyance resistance is generated in the vicinity of the lid portion P2, and the conveyance speed of the envelope P1 in this portion becomes slower than the conveyance speed in other portions. For this reason, the envelope P1 is caught on the first rib 71, and printing on the envelope P1 cannot be performed properly.

On the other hand, as shown in FIG. 5A, the envelope P1
If the distance T1 between the end of the envelope P1 and the upper end of the flat surface 73a is equal to or greater than the width T2 of the lid portion P2 of the envelope P1, the lid portion P2 is the envelope P.
And the second rib 72 does not get caught. That is, in the structure shown in FIG. 5A, the outer surface (flat surface) 73a of the side wall 73 is the side wall 7 in the structure shown in FIG. 5B.
The outer surface 73a of the rib 3 is recessed toward the center of the rib 72, so that the pressing force from the rib 72 is not generated on the lid portion P2. Therefore, the lid portion P2 does not bend when the envelope P1 is conveyed, and printing on the envelope P1 can be performed well.

-Other Embodiments- In the embodiments described above, the case where the present invention is applied to a color ink jet printer having a serial head has been described. The present invention is not limited to this, and may be applied to an inkjet printer provided with a line type head.

The ink lead-out grooves 75 and 76 formed in the ribs 71 and 72 are formed so that their cross-sectional areas gradually decrease toward the downstream side in the recording paper conveyance direction.
The cross-sectional area may be gradually reduced toward the upstream side in the recording paper conveyance direction. In this case, the ink recovery member 8 is arranged at the upstream edge of the platen 7 in the paper transport direction.

Further, the number of ribs 71, 72 and the location of the ribs 71, 72 are not limited to those in the above-described embodiment. For example, means such as forming all the ribs by the second ribs 72 may be adopted.

[0049]

As described above, according to the present invention, ribs are projected on the upper surface of the platen for guiding the recording medium, and the recording medium is conveyed while being supported by the tips of the ribs. The contact between the ejected ink and the recording medium can be avoided. Further, the ink lead-out groove is formed in the rib, and the ink in the ink lead-out groove is discharged from the platen by utilizing the "capillary phenomenon". Therefore, even when the amount of ink ejected onto the platen is relatively large, the back surface of the recording medium is not contaminated by this ink. Further, since the ink can be automatically discharged by the "capillary phenomenon" and a special removing work for removing the ink from the platen is not required, it is possible to provide an inkjet image forming apparatus which is easy to use.

Further, as the shape of the ink lead-out groove, when the groove width dimension orthogonal to the recording medium conveying direction is gradually reduced toward the inner side in the depth direction, it is introduced into the ink lead-out groove. The generated ink can be advanced to the bottom of the ink outlet groove by the capillary phenomenon in addition to gravity.
It is possible to prevent the ink from overflowing the ink lead-out groove. Therefore, it is possible to reliably prevent the ink from adhering to the back surface of the recording medium.

Furthermore, when an ink collecting member for collecting the ink drawn out from the ink drawing groove is provided, the ink that has moved in the ink drawing groove can be collected by the ink collecting member, and the ink drawing groove can be collected. It is possible to prevent the ink discharged from the printer from returning to the platen again, and it is possible to improve the reliability of the means for discharging the ink.

[Brief description of drawings]

FIG. 1 is a diagram showing an internal configuration of a color inkjet printer according to an embodiment.

FIG. 2 is a plan view of a platen.

3 shows a cross-sectional shape of the first rib, (a) is a cross-sectional view taken along the line AA in FIG. 2, (b) is B in FIG.
-B is a sectional view taken along the line B, and (c) is a sectional view taken along the line CC in FIG.

4A and 4B show sectional shapes of second ribs, where FIG. 4A is a sectional view taken along the line DD in FIG. 2, and FIG.
2 is a cross-sectional view taken along the line E, and FIG. 3C is a cross-sectional view taken along the line FF in FIG.

5A and 5B show a state in which the front end of the envelope is conveyed when the front face of the envelope is printed, FIG. 5A shows a case where the front end of the envelope is conveyed by a second rib, and FIG. It is a figure which shows the case where it conveys by.

[Explanation of symbols]

1 Color Inkjet Printer (Inkjet Image Forming Device) 52 Recording Head (Image Forming Unit) 7 Platen 71 First Rib 72 Second Ribs 75, 76 Ink Derivation Groove 8 Ink Recovery Member P Recording Paper (Recording Medium) P1 Envelope (Recording Medium ) P2 lid part (extension part)

   ─────────────────────────────────────────────────── ─── Continued front page    F term (reference) 2C056 EA16 EA27 HA29 HA33 JC15                 2C058 AB18 AC07 AD01 AE02 AF04                       AF20 AF31 DA03 DA11 DB14

Claims (7)

[Claims] 1. An inkjet image forming apparatus, comprising: a platen for guiding a recording medium; and an image forming means arranged to face the platen and ejecting ink droplets onto the recording medium conveyed along the platen. On the surface of the platen, a plurality of ribs extending in the recording medium conveyance direction are provided in a protruding manner, and on the respective ribs, an ink lead-out groove that is recessed in the ink ejection direction and extends in the recording medium conveyance direction is formed, The inkjet image forming apparatus is characterized in that a cross-sectional area of the ink lead-out groove is formed so as to gradually decrease toward at least one of an upstream side and a downstream side in a recording medium conveyance direction. 2. The ink jet image forming apparatus according to claim 1, wherein the ink lead-out groove has a groove width dimension of an open side end portion in a cross section orthogonal to the recording medium conveyance direction, on the upstream side or the downstream side in the recording medium conveyance direction. An inkjet image forming apparatus, which is formed so as to gradually decrease toward at least one of the above. 3. The ink jet image forming apparatus according to claim 1 or 2, wherein the groove width dimension of the ink lead-out groove perpendicular to the recording medium conveyance direction gradually decreases toward the inner side in the depth direction. An inkjet image forming apparatus characterized by being formed. 4. The ink jet image forming apparatus according to claim 1, 2 or 3, for collecting the ink led out from the ink lead-out groove in the vicinity of the end of the ink lead-out groove where the cross-sectional area is the smallest. An ink jet image forming apparatus, wherein the ink collecting member is provided. 5. The inkjet image forming apparatus according to any one of claims 1 to 4, wherein the inner surface of the ink lead-out groove is formed of a material having hydrophilicity or lipophilicity. Inkjet image forming apparatus. 6. The inkjet image forming apparatus according to claim 1, wherein the outer surface of the rib is formed of a material having water repellency or oil repellency. Image forming apparatus. 7. The inkjet image forming apparatus according to claim 1, wherein the recording medium includes an extension portion extending from the end portion in the direction orthogonal to the transport direction to the platen surface side. Inkjet, characterized in that the outer surface shape of the rib is set such that the distance between the end of the recording medium and the outer surface of the rib is equal to or longer than the length dimension in the extension direction of the extension portion. Image forming apparatus.