A device for ensuring a contact between a print medium and a guiding platform at a printing zone of a graphics printer

Abstract: A device for ensuring contact between a print medium provided with a guiding platform (20) at the printing zone of a graphics printer, suction means (26a, 26b) for creating a partial vacuum in a suction chamber (24) located beneath the platform (20) and a plurality of orifices (22) formed throughout the width of the platform (20) communicating with said chamber (24). The print medium can be guided over the platform (20) covering a variable portion of the width of the platform (20) as a fucntion of the format of the print medium. The suction chamber (24) is provided with one or more partitions (30₁ to 30₃) separating the chamber (24) width-wise into compartments (24₁ to 24₄) while leaving a limited passage section between adjacent compartments. A first compartment (24₁) extends beneath a portion of the platform (20) thad comprises at least the fraction of the width of the platform (20) occupied by the print medium having the smallest format amongst the formats accomodated by said printer.

Description: BACKGROUND OF THE INVENTION Field of the invention The present invention relates to a device for ensuring contact between a print medium and a guiding platform at the printing zone of a graphics printer. The present invention more particularly relates to vector mode graphics printers (pen plotters) in which a drawing is produced by relative movement between the print medium and a writing head. The plotter can operate either continuously or sheet by sheet. In the continuous mode, the plotter is fed by a print medium drawn from a supply roller. In the sheet by sheet mode, the plotter is fed by precut sheets. Prior art In a penplotter, such as shown in figure 1, a drawing is produced by a combination of to-and-fro movements of the print medium 10 in a first direction X, and of a writing head 12 in a second direction Y, perpendicular to the first direction. In the active zone of the writing head, the print medium 10 passes over a platform 20 that may be extended at the front and/or the rear by a skirting. The displacement of the print medium in the X direction is e.g. provided by a drum whose upper generatrix is at the level of the surface of the platform and upon which the print medium is applied by means of pressure rollers. The displacement of the writing head 12 is effected, in the Y direction, above and parallel to the platform. At the level of the active zone of the writing head 12 the platform 20 comprises a plurality of orifices 22 which communicate with a suction chamber located beneath the platform so as to create a partial vacuum beneath the print medium. The purpose of this partical vacuum is to prevent the print medium from being lifted above the patform at the level of the writing zone under the effect of the movements imparted to the medium. However, there are certain conditions in which the print medium 10 is not held in contact with the platform 20 with the requried degree of efficiency. Depending on its format, the print medium may occupy a more or less large portion of the width of the platform and will accordingly cover all or only a part of the orifices 22 of the platform. In general, the print medium is brought on the platform with a first edge 10a placed at one end 20a of the platform, irrespective of the print medium format. Consequently, the second edge 10b of the print medium is at a larger or smaller distance from the other end 20b of the platform, leaving a larger or smaller number of orifices 22 uncovered, depending on the width, i.e. the format, of the print medium. Because of leakages through the uncovered orifices, the partial vacuum generated beneath the print medium may be insufficient to ensure a good contact between the print medium and the patform, particularly in the case of small formats. Consequently, occasional risings of the print medium may occur, especially at the level of its edge 10b, which brings about a number of drawbacks. This can indeed result in untimely contacts between the print medium and the writing head while the writing tool carried by the latter is in a raised position. This leads to parasitical lines appearing on the drawing, which is of course highly undesirable. Moreover, it has been observed that the corner of the print medium, at its free end, at edge 20b, can rise off the platform and come into contact with different parts of the printer and in particular into contact with the casing of a cutting device 18 carried by the writing head 12 and intended for cutting off the formats when the printer operates with a continuous print medium. Or, a rubbing of the printed face of the print medium over a part of the printer can smear the drawing, the latter not being dry at that stage. Moreover, if the free end of the print medium comes to block itselfs against a part of the printer, a jamming (accumulation of the print medium) could result through which the operation of the printer could be stopped. In order to ensure the best possible contact of the print medium against the platform, it could be envisaged to increase the power or the number of suction devices (these are generally in the form of one or two fans) used to generate the required partial vacuum in the suction chamber. However, this would result in an increase of the obstruction, cost and energy consumption, as well as in an increased level of noise. OBJECTS AND SUMMARY OF THE INVENTION The object of the present invention is to provide a device capable of ensuring an effective contact of the print medium irrespective of its format, without calling on additional suction means, thus allowing the use of a large number of formats. More particularly, the present invention aims to provide a simple device by means of which leakage effects through the orifices of the platform are minimized when using small formats. This object is achieved by the fact that the suction chamber is provided with one or more partitions separating the chamber width-wise into compartments while leaving a limited passage section between adjacent compartments, the one or first of the compartments extending beneath a portion of the platform which comprises at least the fraction of the width of the platform occupied by the print medium having the smallest format amongst the formats accomodated by the printer. Thus, in the case of a small format, the partitioning of the suction chamber makes it possible to limit leakage effects through the orifices of the platform not covered by the print medium. It is then possible to keep a partial vacuum throughout the width of the print medium sufficient for pressing the latter completely against the platform. The suction chamber can be equipped with a number of partitions placed in the chamber at locations substantially corresponding to the zones of the platform occupied by longitudinal edges of print media in the different formats accomodated by the printer. In order to obtain a suction in the chamber sufficient to ensure satisfactory contact of the print medium,for large formats as well as for small formats, one or a number of suction devices are provided, the suction device or one of the suction devices acting within the first compartment. The first compartment, which is located under a portion of the platform that comprises the fraction of the platform width occupied by the print medium having the smallest format, is positioned at one end of the suction chamber or at its center, depending on whether the print media are brought over the platen with a longitudinal edge located at an end of the platen, or whether the print media are centered on the platen. BRIEF DESCRIPTION OF THE DRAWINGS The invention shall be more clearly understood from the following description of the preferred embodiments given as a non-limiting example with reference to the appended drawings in which: Figure 1 is a very schematic general view of a pen plotter, Figure 2 is a partial view showing a suction chamber partitioned in accordance with the present invention, Figure 3 is a lateral sectional view of the chamber shown in Figure 2, Figures 4A to 4D are diagrams illustrating the operation of the device according to the invention shown in Figure 2, Figure 5 illustrates the partial vacuums measured at the sheet edge for different formats with the suction chamber shown in Figure 2, and Figure 6 is a highly schematic representation of an alternative embodiment of a device according to the invention. DESCRIPTION OF THE PREFERRED EMBODIMENT An embodiment of the present invention is described herinafter in the context of a pen plotter of the type illustrated in Figure 1, which uses a continuous print medium. Reference shall initially be made to Figures 1 to3. The pen plotter comprises a platform 20 over which is guided the print medium 10 which is drawn form a supply reel 11. As already indicated, the print medium 10 is displaced in the X direction (longitudinal direction of the print medium) by means of a drum 14 having a horizontal axis perpendicular to the X direction. The print medium is applied against drum 14, in the region of its edge portions 10a, 10b, by means of pressure rollers 16a, 16b which act through a transversal slot 23 formed across the platform 20. The upper generatrix of drum 14 lies substantially at the level of platform 20, through slot 23. The print medium 10 has a longitudinal edge 10a located close to the end 20a of the platform, irrespective of its format. The pressure roller 16a acting at the edge 10a accordingly has a fixed position in the Y direction. The other longitudinal edge 10b of the print medium is more or less remote from end 20a, depending on the format of the print medium. Consequently, the pressure roller 16b has an adjustable position in the Y direction, in order to be brought to the edge 10b. The writing head 12 is movable in the Y direction along a writing line 21 situated at the surface of the platform 20, slightly to the front of the slot 23. The writing line 21 may be materialized by a groove formed in the upper surface of the platform over a part of the thickness thereof. The writing head 12 bears a writing tool 12a. Several writing tools may be stocked in a turret 13 at the end 20a of the platform, so allowing an automatic changeover of the writing tool. As shown in more detail in Figures 2 and 3, the suction chamber 24 is comprised of a box section whose upper wall is formed by the platform 20. The latter is provided with orifices 22 along its entire width. The orifices are arranged in a number of rows situated in the vicinity of the writing zone and slot 23. More specifically, Figure 2 shows rows of orifices situated behind the slot 23, between the latter and the writing line 21, and in front of the writing line. Other orifices may be formed at the bottom of the groove defining the writing line. The partial vacuum inside the chamber 24 is created by means of fans 26a, 26b. The air expulsed by the fans is directed to a conduit 28 (shown only in Figure 3) which is located beneath the suction chamber and discharges at the front of the printer, at a level where the print medium 10 leaves the platform 20 and is no longer supported. The conduit 28 extends practically throughout the width of platform 20. Accordingly, the air flux 29 drawn out by conduit 28 produces a secondary guiding of the print medium by acting on the portion 10c of the print medium that hangs at the front of the printer, to prevent it from coming into contact with elements of the printer or objects supported by the latter. Such a secondary guiding system is the subject of Patent Application FR 89 17469 filed on December 29, 1989 by the Applicant. To ensure a sufficient air flux 29 even when orifices 22 are all -or practically all-covered by the print medium (maximum format), a supplementary air inlet is provided in the suction chamber 24. This air inlet is e.g. constituted by an opening 27 (shown only in Figures 4A to 4D) situated in the lateral wall of chamber 24 at the side of the end 20b of the platform. According to the invention, the suction chamber 24 is partitioned along its length. By way of example, we consider a plotter that uses print media adapted to produce drawings ranging in formats from A4 (210x297 mm) to A0 (841x1189 mm). In this case, preferably a number of partitions are arranged inside of chamber 24, and specifically at locations corresponding to the zones of the platform situated at distances from the end 20a of the platform that are substantially equal to the widths of the print media used for the smallest drawing formats. Thus, a first partition 30₁ is situated at a location corresponding to a zone of the platform situated at a distance from the end 20a substantially equal to the width L₁ of the print medium used for printing an A4 format (L₁ = 297mm). Likewise, a second partition 30₂ is situated at a location that corresponds to a zone of the platform situated at a distance from the end 20a substantially equal to the width of L₂ of the print medium used for printing an A3 format ( L₂ = 420mm). Finally, a third partition 30₃ is situated at a location corresponding to a zone of the platform situated at a distance from the end 20a substantially equal to the width L₃ of the print medium used for printing an A2 format (L₃ = 594mm). The partitions 30₁, 30₂,30₃ are formed by vertical plates that extend in the X direction and separate the chamber 24 into compartments 24₁, 24₂,24₃ and 24₄. Each partition does not completely isolate one from the other the two ajacent compartments that it delimits, but leaves a limited passage section therebetween. As shown in Figure 3, this passage section can be in the form of a clearance between the upper edge of the partition and the inner face of the platform. Figure 3 also shows a cutting formed in the upper edge of the partition to allow the passage of the drive drum 14. The purpose of the partitions 30₁, 30₂,30₃ shall be described hereafter with reference to Figures 4A to 4D. When the print medium used has a minimum width, it extends on the platform between the edge 20a and the level of partition 30₁, covering the orifices 22 that communicate with compartment 24₁ (Figure 4A). Due to the provision of partition 30₁, the effect of the leakages through the orifices 22 not covered by the print medium and through opening 27 is minimized, because the partition 30₁ leaves a limited passage section between compartment 24₁ and the remainder of the suction chamber. The fans 26a and 26b are located inside compartments 24₁ and 24₂ at the ends of chamber 24. In the example illustrated by figure 4A, the fan 26a produces the partial vacuum necessary to maintain the print medium on the platform, while fan 26b serves essentially to provide air flux 29. For a given power of fans 26a and 26b, the passage section between compartments 24₁ and 24₂ is determined in order that the edge 10b of the print medium remains pressed against the platform. It has been observed that a partial vacuum of approximately 4 to 4.5 mm H₂O (i.e. 40 to 45 Pa) at the level of edge 10b of the print medium is sufficient. The passage section is therefore selected to ensure the required level of partial vacuum. When the print medium used coresponds to an A3 format drawing, it extends on the platform between edge 20a and the level of partition 30₂, covering the orifices 22 that communicate with compartments 24₁ and 24₂ (Figure 4B). Partition 30₂ leaves a limited passage section between compartments 24₂ and 24₃ to ensure the required partial vacuum at the level of edge 10b of the print medium. The contact of the print medium on the platform is ensured by fan 26a located inside compartment 24₁. Since the partition 30₁ creates a drop of load, the passage section at the level of partition 30₂ may become smaller than the one existing at the level of partition 30₁ to ensure the required partial vacuum at the level of partition 30₂. Likewise, when the print medium corresponds to an A2 format drawing, it extends on the platform between edge 20a and the level of partition 30₃, covering the orifices 22 that communicate with compartments 24₁, 24₂ and 24₃ (Figure 4C). The passage section between compartments 24₃ and 24₄ is set so as to ensure the required partial vacuum at the level of edge 10b of the print medium. The passage section may become smaller than those left by the partitions 30₁ and 30₂ by virtue of the load losses created by the latter. When the print medium corresponds to a drawing having an A1 or A0 format, it extends on the platform covering the orifices that communicate with compartments 24₁, 24₂,24₃, as well as a part of or all of the orifices that communicate with compartment 24₄. The partial vacuum at edge 10b of the print medium is produced by fans 26a and 26b (Figure 4D). Tests have been conducted with a graphics printer equipped with a suction chamber having three partitions arranged as described above. The platform showed 329 circular suction orifices 22 having a diameter of 4 mm. The partitions were identical, leaving the same passage sections, each equal to 300 mm², between adjacent compartments. Figure 5 shows the partial vacuums measured at the level of edge 20b for different widths of the print medium. Here the fans 26a, 26b used were identical and produced a static pressure of 70 Pa and a flowrate of 47 l/s at a zero pressure drop. As can be seen from Figure 5, for all formats, an effective contact of the print medium against the platform is ensured since the partial vacuum beneath edge 20b is always at least equal to 4 mm H₂O (40 Pa). In the foregoing embodiment, the partitions were placed at distances from the edge 20a of the platform corresponding more or less exactly to the widths of the print media. A positioning tolerance is nevertheless possible, so long as the distance at which a partition is situated, is greater than the corresponding width of the print medium, if it is not exactly equal to the latter. Furthermore, it may not be necessary to provide three partitions. In particular, acceptable results have been achieved by leaving out partion 30₁ corresponding to an A4 format, and by determining the passage section left by partition 30₂ so as to obtain the required partial vacuum for the A4 format. More generally, the number of partitions can vary as a function of the number of different formats that can be accomodated by the printer. The foregoing concerns cases where the print medium, irrespective of its format, is always placed on the platform with the edge 10a situated at the end 20a of the platform. It is also possible to bring the print medium centered on the platform, that is to say symmetrically with respect to a central plane of the platform parallel to the X direction. In this case, the partitions for the suctions chamber 24 need to be arranged as shown in Figure 6, that is to say symmetrically with respect to the above mentioned central plane P. Two partitions 30₁, 30₂ that are mutually symmetrical with respect to the plane P delimit a central compartment 24₁ which extends over a part of platform 20 substantially corresponding to the fraction of the width of the latter occupied by the print medium used for producing drawings having the smallest format (A4). The partitions 30₁, 30₂ are thus spaced apart from each other by a distance approximately equal to L₁. The partitions 30₁, 30₂ each leave a limited flow section with the remaining of chamber 24. Two orther partitions 30₃,30₄ that are mutually symmetrical with respect to plane P delimit compartments 24₂,24₃, with partitions 30₁, 30₂ respectively. Partitions 30₃,30₄ are spaced apart from each other by a distance approximately equal to the width L₂ of the print medium used for producing A3 format drawings. Finally, two partitions 30₅, 30₆ that are mutually symmetrical with respect to plane P delimit compartments 24₄, 24₅, with partitions 30₃,30₄ respectively as well as compartments 24₆, 24₇ located at the ends of the chamber. The partitions 30₅, 30₆ are spaced apart from each other by a distance substantially equal to the width L₃ of the print medium used for producing A2 format drawings. The partitions 30₃, 30₄, 30₅ and 30₆ each leave a limited passage section between the adjacent compartments that they separate. The suction in chamber 24 is produced by means of a single fan 26 located in the central compartment 24₁. Other fans may be provided in other compartments, for example inside the end compartments 24₆, 24₇. More generally, the positioning of the partitions in the chamber shall be determined so as to form at least one compartment extending beneath a portion of the platform that comprises at least the fraction of the width of this latter occupied by the print medium used for producing drawings having the smallest format.

Claims: Arrangement for ensuring contact between a print medium (10) and a guiding platform (20) at the printing zone of a graphics printer, comprising suction means (26a, 26b) for creating a partial vacuum in a suction chamber (24) located beneath said platform, and a plurality of orifices (22) formed throughout the width of said platform and communicating with said chamber, said print medium being guided over said platform while covering a variable portion of the width of said platform as a function of the format of the print medium, characterized in that said suction chamber (24) is provided with one or more partitions (30₁ to 30₆) separating said chamber width-wise into compartments (24₁ to 24₇) while leaving a limited passage section between adjacent compartments, the one or first one of said compartments (24₁ ) extending beneath a portion of said platform (20) that comprises at least the fraction of the width of said platform occupied by the print medium having the smallest format amongst the formats accommodated by said printer. Arrangement according to claim 1, characterized in that said suction chamber (24) is provided with a plurality or partitions (30₁ to 30₆) placed within said chamber at locations substantially corresponding to the zones of said platform (20) occupied by longitudinal edges of print media in the different formats accomodated by said printer. Arrangement according to claim 2, characterized in that said passage sections between adjacent compartments have different values that decrease going from said first compartment. Arrangement according to any one of claims 1 to 3, in which said suction means comprise one or more distinct suction devices (26a, 26b; 26), characterized in that said first compartment (24₁) is equipped with the or a suction device. Arrangement according to any one of claims 1 to 4, characterized in that said first compartment (24₁) is located at an end of said suction chamber. Arrangement according to any one of claims 1 to 4, characterized in that said first compartment (24₁) is located at the center of said suction chamber.

Inventors: LEJAOUDOUR THIERRY, LEJAOUDOUR, THIERRY
Applicants: OCE GRAPHICS FRANCE, OCE GRAPHICS FRANCE S.A.
Application year: 1991
Main classification: G01D15
IPC codes: G01D15