CN1890107B - Printing moving substrates - Google Patents

Abstract

An ink jet printing apparatus for printing on a moving substrate (16) is described. The apparatus comprises ink jet printheads (4) for printing on a surface of the substrate, a plurality of rollers (12, 14) arranged to move the substrate (16) relative to the printheads 4, and a pressure source (22). The pressure source (22) is arranged to apply a negative gauge pressure (32) to the substrate (16). The application of the pressure can hold the substrate flat and can reduce undesired movement of the substrate relative to the rollers (12, 14). The apparatus allows for high quality full colour images to be printed onto substrates in a single pass using an ink jet printer.

Description

Print mobile substrate

technical field

The present invention relates to printing. The present invention finds a particular application, but not limited to, printing on moving substrates. The preferred examples relate to inkjet printing, and in particular to transport systems and methods for moving substrates in inkjet printers.

Background technique

The following description refers to the example of single-pass inkjet printing: the substrate is only allowed to pass once through the printhead arrangement.

Inkjet printing has certain advantages over other printing methods, such as allowing rapid changeover of images. However, it has trouble printing high-quality images.

Conventional printers (such as lithography and flexography) are capable of printing high-quality full-color images on continuous fabrics and paper, however, this has been somewhat difficult to achieve with existing inkjet printing technologies.

In the process of inkjet printing, for example, a desired image is formed by attaching a group of ink droplets on the surface of a substrate according to a certain pattern. Ink droplets are typically ejected from an array of nozzles in an inkjet printhead. Relative motion between the inkjet printhead and the substrate is often necessary during the process of printing the desired full image onto the substrate. In some devices, this motion is achieved while the nozzles of the printhead are ejecting ink drops; alternatively, or in addition, printing may also be paused during the motion. Due to the relative motion between the print head and the substrate during the printing of the image, alignment errors will result in a reduction in image quality.

There are also special issues with printing halftone images. Due to the moiré effect in halftone printing, small misregistration between colors can cause severe color shifts. A moiré effect is a pattern, such as a herringbone pattern, that occurs when two or more geometric patterns such as a grid are superimposed. During inkjet printing, a moiré effect occurs when two or more grids of printed ink drops are not properly aligned, for example between the different colors of a color image. This problem also occurs when printing monochrome images with several print heads spaced apart in the printing direction.

In the traditional printing process, it can be controlled by rotating the screen of different printing colors, and at this time, small size moiré rings are produced that are not considered to be visually uncomfortable. However, this technique is not possible with inkjet devices, especially with single-pass devices, because the array of nozzles produces an image directly from a single movement of the substrate.

Therefore, for inkjet printing, it is considered necessary to control the lateral relative motion between the substrate and the printhead with high precision, especially for single pass printing. Typically, lateral movement of 20% of the dot pitch of the printed image will produce unacceptable banding during printing. Therefore, for an image with a resolution of 200dpi, a misalignment of 0.001 inches will result in banding.

A particular problem arises when the substrate is in motion during the printing process, since unwanted movement of the substrate will cause alignment errors and degrade the print quality.

Substrates for printing, such as rigid boards, are usually transported on conveyor belts. However, it is difficult to construct a conveyor belt system that can provide the required motion precision, and substrates can slip on the conveyor belt.

Some existing inkjet printers use a vacuum combined with conveyor belts and rollers to transport substrates to be printed, see Figures 9a and 9b of JP10-315551. This system is designed to detect the substrate from the print of the print swathe and to control the substrate while the printhead is firing ink. A vacuum can be used to hold the substrate on a roller or conveyor to maintain an even gap between the printhead and the surface of the substrate. The linear speed of substrate movement is very low, typically 0.1m/s, due to the small retrieval distance between print packs, typically 1cm. Such systems can only be used to process flexible substrates such as paper. Single-pass printing at speeds exceeding 1m/s on rigid materials such as corrugated boards requires a different approach.

Contents of the invention

A feature of the present invention is to seek to alleviate one or more of the above-mentioned problems.

According to a first aspect of the present invention, there is provided an inkjet printing device capable of printing on a moving substrate. The printer includes: a plurality of inkjet printing heads for ejecting ink to the surface of the substrate, wherein the printing heads are suitable for stationary; a plurality of rollers arranged to move the substrate relative to the printhead; a pressure source arranged to apply negative gauge pressure to the substrate, preferably arranged to retain the substrate on the rollers. Preferably, the application of pressure helps to keep the substrate flat, and/or reduces undesired movement of the substrate relative to the rollers.

Preferably, the negative gauge pressure provides a "hold down" effect on the substrate, increasing the adhesion of the substrate to the rollers, thus reducing undesired movement of the substrate.

By applying negative gauge pressure, substrate slippage on the rollers will be reduced or eliminated. Slippage can occur in other ways, such as in the case of sudden changes in the motion of the rollers as they are driven by stepper motors. The reduction in slippage would be particularly beneficial for substrates having a low coefficient of friction, such as substrates comprising plastics such as polypropylene.

Preferably, the substrate is conveyed directly on the rollers, eg such that the substrate directly contacts the rollers.

The use of a vacuum can also work on flattened substrates. In this way, a controlled gap (eg about 2 mm) can be maintained between the print head and the substrate surface. If the substrate is uneven, it will hit the printhead, smear ink and damage the printhead.

The "hold down" action will also reduce the effect of undesired external forces acting on the substrate, especially if the substrate is susceptible to movement by nearby air currents, such as low quality substrates.

The "hold down" effect provided by the application of negative gauge pressure is also advantageous because the "hold down" effect is provided without special consideration to the edge profile of the substrate, and the device can be used in various shapes and sizes with little modification. Print on substrate.

Preferably, the substrate is generally flat.

It is also advantageous for the rollers, for example, to be able to be manufactured with high precision and mounted precisely on the bearings so that there is little unwanted lateral movement. This also ensures that the rollers turn at the same speed, reducing the risk of loss of traction between the rollers and the substrate, which would cause twisting of the substrate and consequent lateral movement of the substrate.

Preferably, the roller is arranged to support the substrate with its upper surface. Typically, the rollers are generally cylindrical. The outer surface of the roller will be shaped as described below. Preferably, the rollers are arranged substantially parallel, and when the rollers rotate, move the substrate generally in a linear direction. Such rollers are very suitable because they can be used both to support the substrate (e.g. simply placed on top of the roller) and to move the substrate in a desired direction, e.g. by turning the rollers simultaneously or otherwise, This rotation provides movement of the substrate.

Alternatively, or in addition, the substrate may also be steered in other directions during printing. The application of the vacuum allows different orientation changes, so for example the substrate can be controlled from above while the printhead is pointed upwards towards the underside of the substrate, or the rollers can form a vertical plane while the substrate is moved vertically or horizontally.

Preferably, the plurality of printheads comprise more than one row of print nozzles. The printheads can be arranged independently or combined into one or more printhead assemblies.

Preferably, the apparatus is adapted to print on a plurality of discrete substrate surfaces. Preferably, the device is adapted to print on sheet-like material. Such devices are distinguished from those printers which are adapted, for example, to print on continuous textile material.

Although the device is suitable for printing on sheet-like materials, it should be noted that substrates may also come in different forms. In general, substrates of any shape can be transported in the printer using the method described here; this method is particularly suitable for substrates with at least one flat surface on which negative gauge pressure can be applied.

Precise positioning of discontinuous sheets of material is particularly difficult to maintain using conventional methods, and the "hold down" action is particularly advantageous in this case.

The roller can be made of any suitable material and can further include a high friction outer surface to increase the friction between the roller and the substrate.

Preferably, the apparatus comprises at least three rollers arranged to move the substrate relative to the printhead. It has been found that in many applications at least three rollers are required to provide adequate control of the movement of the substrate. The number of rollers and their dimensions depend on the size, shape and weight of the substrates they are to move. It is contemplated that at least four and more preferably at least five rollers will be used. In the preferred embodiment, ten rollers are used.

Preferably, the roller is arranged with its axis of rotation substantially perpendicular to the printing direction. Preferably, the rollers are arranged close to each other, preferably directly adjacent to each other, preferably in the printing direction. Preferably, the printing direction coincides with the moving direction of the substrate.

The combination of the vacuum "holding down" and the rollers that move the substrate provide tight control over the substrate movement, thus improving the accuracy of ink drops onto the substrate, reducing streaking defects and producing higher quality images. Preferably, the apparatus also includes a controller for controlling the movement of the rollers. Preferably, the controller is adapted to control the relative movement between the substrate and the printhead.

Preferably, the arrangement is such that the relative displacement of the substrate perpendicular to the printing direction is within a predetermined tolerance.

It will be readily appreciated that, preferably, the speed of the rollers is carefully controlled. The difference in the speed of the rollers relative to each other will cause the two substrate pulling surfaces (moving surfaces) to move at different linear velocities within the plane of the substrate. This causes distortion of the substrate as it moves along the rollers. Preferably, the speed difference is reduced by controlling the rotation of the rollers so that the speed difference between them is zero or close to zero.

Preferably, the rollers are mounted substantially parallel to adjacent rollers, preferably at an angle between adjacent rollers and parallel of no more than 6 milliradians, preferably no more than 4 milliradians, more preferably no more than 2 milliradians.

Preferably, the rollers are rotatable, and preferably the device is adapted to control such rotation so that the peripheral speed of adjacent rollers (preferably the speed of the outer surface of the roller controlling the substrate) is controlled within 1%, preferably Within 0.5%, preferably within 0.3%.

It is especially important to control the movement of the substrate in the vertical direction or lateral direction of printing, because even small printing misalignment can be easily seen in this direction.

Preferably, the selection of the predetermined tolerance depends on the visually acceptable image quality and the nozzle pitch of the print head in the relevant direction.

Preferably, the relative displacement is less than 30% of the dot pitch of the printed image, preferably less than 20% of the dot pitch, more preferably less than 15% of the dot pitch, most preferably less than 10% of the dot pitch.

Preferably, negative gauge pressure is applied to the area between adjacent rollers on the substrate. Such a setup allows better control of the substrate.

Preferably, negative gauge pressure is applied to the substrate only in the area between adjacent rollers. Preferably, the negative gauge pressure is not applied through the roller itself, for example through perforations provided in the roller.

The number of vacuums applied to the substrate is selected according to the nature of the substrate and the desired print quality. It is easy to understand that high pressure will hold the substrate down better, thus reducing potential slippage and resulting in better print quality, but will also cost more and run a greater risk of damaging the substrate, e.g. bending.

Preferably, the apparatus further comprises an element arranged between the rollers adjacent to the substrate; preferably, this element is arranged to restrict the air flow between the rollers.

The use of a masking element can throttle the airflow through the gap between the rollers, thus reducing the size of the vacuum pump required. In addition, it avoids excessive air flow near the substrate, which interrupts the ejection of ink droplets from the printhead. Additionally, pressure can be applied to the substrate more rapidly.

For porous substrates, the mask element is particularly desirable because it is able to concentrate the vacuum action away from the middle of the substrate between the rollers to the area of the substrate close to the rollers where the vacuum action is required.

Alternatively, or in addition, this element is also arranged to reduce deformation of the substrate between the rollers. This would include a different element than the mask element, or the same element, serving more than one function.

When the substrate is mounted on the rollers, there will be an area between the rollers where the roller is not in contact with the substrate. Preferably, this is the area where the vacuum is applied. In areas where the substrate does not have considerable stiffness, the action of the vacuum may cause deformation of the substrate in that area. This element will be arranged to support the substrate as it passes between the rollers. Especially advantageous for porous substrates. It is easy to understand that the support element also has the additional function of restricting the air flow as described above.

In other arrangements, however, there is preferably a gap between the substrate and the element. This is especially necessary when substrates are moving rapidly through the apparatus.

Preferably, the device further comprises guiding means for guiding the leading edge of the substrate. This may be provided as a function of the elements described above. Preferably, the guide means are located between the rollers. The aforementioned elements may provide a guide surface for guiding the advancing substrate. Such guides also help prevent puckering of the material around the rollers by guiding the leading edge of the substrate.

Preferably, this element and/or guide means consist of a hard material with a surface having a low coefficient of friction.

It will be readily appreciated that a wide range of substrates can be printed using the apparatus described herein. This device is especially suitable for substrates made of hard or rigid materials. Such materials are less likely to be deformed or damaged when subjected to vacuum and/or passing through rollers. Preferably, the substrate is constructed of a substantially rigid material. In a preferred arrangement, the substrate consists of a rigid flat plate. Substrates can include paper, card, wood, metal, plastic or ceramic materials.

When printing rigid substrates, preferably the outer surface of the roller comprises a deformable material, such as an elastomer.

Preferably, the arrangement is such that the substrate is mounted on the deformable surface during printing. In this way, since the deformable surface conforms to the surface of the substrate, the downward holding of the substrate can be improved when vacuum is applied. The outer surface of the roller may consist of elastomer.

When printing less rigid substrates, rollers with shaped outer surfaces, preferably with corresponding guides, can be used to reduce the risk of the substrate being caught in the gap between the guides and the rollers.

Preferably, the leading edge of the guiding element is not straight.

Preferably, it can also be arranged so that there is no need for rollers in contact with the surface of the substrate to be printed. Such an arrangement is possible due to the application of negative pressure as described here.

Preferably, the printer is adapted to print substrates in a single pass. Preferably, the printer can print the desired image with a single pass of the substrate over the printer. Compared with multi-pass printing, single-pass printing can greatly reduce printing time. However, this printing technique requires exceptional precision in securing the substrate position if printing defects are to be minimized.

For multi-pass printing, the substrate is usually mounted on a reciprocating table; the trajectory of the table is precisely controlled. This setup is not feasible for single-pass printing because it would take a long time to load and unload the stage.

Preferably, the print head is stationary during printing. In a preferred arrangement, the print head is stationary relative to the roller. This arrangement provides a reference point for controlling the movement of the substrate as it moves along the rollers. If print defects are to be minimized, a fixed print head is used, again requiring particularly high precision in securing the substrate position. Any unplanned movement of the substrate relative to the simultaneous movement of the stationary printhead during the time interval between drop ejections is likely to cause subsequent ink drops to accumulate far from their intended location, thus causing subsequent Improper alignment of ink drops. If the alignment of the substrate continues to be incorrect, the pattern printed by the inkjet printhead will be misregistered.

The printhead is stationary while ejecting the ink drops; in certain preferred examples, the printhead does not move during printing of the substrate. In some instances, the printhead does not move within the plane of the surface to be printed at all, but can be adjusted in height.

Preferably, the substrate moves when ink droplets are ejected. Preferably, the movement speed of the substrate relative to the print head is greater than 0.5 m/s, preferably greater than 1 m/s, preferably in the printing direction. Preferably, the substrate is moved at a speed greater than 0.5 m/s, preferably greater than 1 m/s.

Preferably, the system is adapted to print color images. The system may be suitable for printing halftone images. For this type of image, the correct placement of the ink drops on the substrate is more critical if acceptable print quality is to be achieved. In particular, for color, a color image is formed by sequentially printing ink droplets of different colors onto the area to be printed; if there is unnecessary movement of the substrate between printing different colors, it will greatly degrade the image quality. The same effect can be produced if multiple print heads are used to print monochrome images, for example to improve addressability.

The use of multiple print heads can provide high-quality printed images. However, if there is unwanted movement of the substrate between prints by different printheads, this will greatly degrade the quality of the image.

Preferably, the device comprises a drop on demand printhead.

Preferably, the device is adapted to print images with a resolution greater than 120 dpi. Preferably, in a direction perpendicular to the printing movement direction (printing direction), the resolution of the printed image is greater than 120 dpi. Preferably, the resolution in the printing direction is greater than 120 dpi. Preferably, the resolution is greater than 150dpi.

Another aspect of the present invention provides a transport apparatus for moving a substrate past a printhead in an inkjet printer, the apparatus comprising: a plurality of rollers arranged to move the substrate relative to the printhead; a pressure source, wherein the pressure source is configured To apply negative gauge pressure to hold the substrate on the rollers.

Another aspect of the present invention provides a method of printing a substrate in an inkjet printer comprising a plurality of print heads, a plurality of rollers and a pressure source, the method comprising the steps of: and applying a negative gauge pressure on the substrate to hold the substrate on the roller, wherein the printhead is stationary during ejection of ink onto the substrate. Preferably, the substrate is printed in a single pass.

The present invention also provides a printing substrate using the printer described herein and/or the printing method described herein.

A further aspect of the invention provides an apparatus substantially as herein described with reference to and as shown in the accompanying drawings, and a method substantially as herein described with reference to and with reference to the accompanying drawings.

Features of one aspect of the invention may be provided by features of other aspects in appropriate combinations.

Description of drawings

Only adopt the method for example below, introduce preferred feature of the present invention with reference to accompanying drawing, wherein:

Figure 1 schematically shows an inkjet printer comprising a transport system;

Figure 2 shows a part of the delivery system shown in Figure 1;

Figure 3 shows a schematic top view of a part of the delivery system shown in Figure 1;

Figure 4 is a perspective view of a portion of the delivery system shown in Figure 1;

Fig. 5 schematically shows the pressure system of the conveying system shown in Fig. 1;

Figure 6 schematically shows a print head arrangement;

Figure 7 shows an alternative roller and mask element arrangement; and

Figure 8 shows yet another alternative roller and mask element arrangement.

Detailed ways

Figure 1 schematically shows an inkjet printer 2 having a stationary printhead assembly 4 which is placed on a continuous rigid sheet substrate 16 as the substrate is moved across the printer by a conveyor system 10 past the printhead assembly 4, 16', 16" ink drops are printed. For single-pass printing, the substrate moves across the printer only once through the printhead.

The first conveyor 6 moves the unprinted substrate 16' towards the conveyor system 10; the second conveyor 8 moves the printed substrate 16" away from the conveyor system; the conveyors 6, 8 include conveyor belts, and of course other devices can be used. Substrates are loaded and unloaded from the conveyors 6, 8 by conventional methods.

The transport system 10 is arranged to provide precise movement of the substrate 16 through the printhead arrangement 4 . The length of the transport system 10 in the printing direction and the width of the transport system perpendicular to the printing direction are selected so that the entire substrate 16 is supported by the transport system when the image is printed.

The print head unit 4 includes four sets of print heads, each of which prints a different color selected from cyan, magenta, yellow and black to form a full-color image on the substrate. The print head assembly 4 does not move during printing. However, the print head can be moved up and down to accommodate substrates of different thicknesses.

In this embodiment, the substrate 16 is a corrugated rectangular flat cardboard approximately 2mm thick. However, the apparatus described can be used to print on widely different types of substrates, such as carton board, plastic sheets such as PVC, and on substrates of different shapes.

2 and 3 are schematic side and top views, respectively, of a portion of the conveyor system 10 . The conveyor system 10 in this example includes ten cylindrical rollers. For the sake of clarity, only two rollers 12, 14 are shown in Figures 2 and 3 . Figure 4 shows four rollers in a perspective view. The rollers 12 and 14 are arranged side by side in parallel such that there is a uniform gap between the rollers which forms a cavity 28 between a pair of adjacent rollers. The base plate 16 is supported on roller contact surfaces 18 , 20 .

The transport system 10 moves the substrate 16 in the printing direction 30 (right to left in FIG. 3 ) by rotating the rollers 12 , 14 in a counterclockwise direction 38 , 40 . A few of all the rollers can be driven and the others turn freely. Alternatively, all rollers can be driven simultaneously by the roller drive system. In this example, the arrangement includes a gear train and a single motor to drive all ten rollers, although other arrangements could be used, such as separate direct drive motors for each roller.

The rollers 12, 14 rotate in a counterclockwise direction 38, 40 at a fixed or variable controlled angular velocity. In this example, the rollers were rotated at 300 rpm such that the linear speed of the substrate on the rollers was only slightly greater than 1 m/s. The axes of rotation of the rollers are substantially parallel. The speeds of motion of the rollers along the contact surfaces 18, 20 are substantially equal to minimize any unbalanced forces acting on the substrate that could cause undesired lateral movement of the substrate.

The roller is cylindrical in shape and its length is long relative to its diameter. The rollers are made of steel. Other suitable rigid materials may also be used, such as plastic or polymeric materials, or other metals.

The transfer system 10 also includes a pressure system 22 for controlling the substrate 16 on the rollers 12, 14 to keep the substrate flat and reduce substrate slippage. The pressure system 22 is arranged to apply a negative gauge pressure 32 to the cavity 28, and thus to the substrate, whereby the substrate 16 is held against the contact surfaces 18, 20 of the rollers.

Thus, unwanted movements of the substrate 16, especially in the transverse direction 36 (perpendicular to the printing direction 30 in the plane of the substrate 16) can be reduced. Movement of the substrate in the lateral direction causes print alignment errors during printing and is therefore very undesirable.

To reduce lateral movement of the rollers, the rollers are mounted with deep groove ball bearings at one end of the rollers to minimize axial movement of the rollers. Roller bearings are used at the other end to avoid over-constraint.

As shown in FIG. 5 , the pressure system 22 includes a fan 24 . The system 22 comprises a housing 26 surrounding the lower surface of the roller arrangement. A fan 24 is mounted on the bottom of the housing and is arranged to expel air from the housing 26 so as to create a vacuum on the lower surface of the roller arrangement. In this example, the duct exhaust fan ACH315-12A is used.

The applied vacuum is selected to provide the desired hold down onto the rollers for a particular print substrate. In this example, the substrate was corrugated cardboard of 2mm thickness and the pressure used was equivalent to 50mm of water column.

The mask element 100 is positioned between a pair of adjacent rollers 12,14. Vacuum port 26 is disposed below mask element 100 . The mask element has several functions: throttles the airflow generated by the vacuum through the gap 102 between the mask element and the roller contact surfaces 18, 20, thereby limiting the size of the vacuum pump required; guides the leading edge of the substrate 16, to help prevent the substrate from wrapping around the rollers; supporting the substrate between the rollers reduces the risk of substrate bending. When the substrate is porous, the mask element 100 is configured to concentrate the pressure on the substrate at the top of the rollers, further reducing substrate bowing.

In this example, the mask element 100 is made of stainless steel and has a T-shaped profile in cross-section. The arms of the element provide guiding surfaces for the substrate 16 .

In this example, the diameter D of the rollers is 75 mm and adjacent rollers are mounted with a distance A between the axes of rotation of 100 mm. The gap 102 between the arms of the mask element 100 and the roller surface 20 is 1.5 mm.

The upper plane 104 of the mask element 100 also provides a guiding surface for the leading edge of the substrate as it moves from one roller to the other. In this example, the mask element 100 is mounted such that the distance C of the upper plane 104 below the top of the roller is 1.5 mm. This reduces the risk of damage to the substrate at the leading edge of the mask element 100 .

FIG. 3 shows an image 109 printed by the print head arrangement 4 . The print head device 4 shown in FIG. 6 includes 16 print heads 105, each containing cyan 110, magenta 112, yellow 114, and black 116, for a total of 64 print heads. Groups of four printheads containing each color are arranged side by side so that the entire width of the substrate surface can be printed in a single pass of the substrate. Here, the print width is 260mm. In this example, the print head is a Spectrum SE print head, the printing resolution is 50dpi, and each color has 4 depths, so the printing addressability is 200dpi.

The device uses an inkjet printer to print high-quality, full-color images on a substrate in a single pass.

It will be understood that the foregoing has described the invention by way of example only and that modifications of detail may be made within the scope of the invention.

For example, if the printing substrate has a rigid lower surface, the outer surfaces of the rollers 18, 20 may comprise a deformable material, such as foam or rubber, that deforms when negative gauge pressure is applied to the substrate to help hold the substrate down on the rollers .

When printing flexible thin substrates using the setup described above, there is a risk of dragging the substrate around the rollers and either blocking the gap between the rollers and the mask element, or sending the substrate directly into the vacuum zone. This can limit the thinness and flexibility of substrates that a particular device can handle. Figures 7 and 8 show yet another embodiment suitable for more flexible substrates.

Referring to Figure 7, the roller 200 has a non-uniform radius along its length, forming a toothed outer surface. Mask/guide elements 215, 220 arranged close to the rollers are formed in the shape of teeth 230 corresponding to the reduced radius regions of the rollers. This way the leading edge of the mask/guide element is not straight, which reduces the risk of the substrate passing through the rollers and the guide element.

FIG. 8 shows yet another example where tooth extensions at the leading edge of the guide/mask elements connect to adjacent guide/mask elements to form guide/mask grid 350 above roller 300 . This further reduces the risk of damage to the substrate on the guide/mask element.

It is conceivable that negative gauge pressure could be applied to the substrate from a roll with an internal vacuum chamber. In this case, a substantially hollow roller may be used, which has a perforated skin through which the vacuum is applied.

Each feature disclosed in the specification, claims (where appropriate) and drawings may be independent or in any suitable combination.

Claims (14)

1. An inkjet printing device for printing on a substrate, the device comprising: a printhead arrangement comprising a plurality of inkjet printheads for ejecting ink droplets onto the surface of the substrate, wherein the printhead arrangement is adapted to hold the printheads substantially stationary as ink droplets are ejected from the printheads move; a plurality of rollers configured to move the substrate past the printhead during ejection of ink drops from the printhead; a pressure source, wherein the pressure source is configured to apply a negative gauge pressure to the substrate to hold the substrate on the roller in the region of the printhead arrangement; a mask element positioned between a first roller and a second roller of the plurality of rollers, thereby defining a first gap between the mask element and the first roller and between the mask element and the first roller. A second gap between the two rollers, wherein the mask element restricts the airflow generated by the vacuum through the first gap and the second gap. 2. The inkjet printing device of claim 1, wherein the inkjet printing device is adapted to print on the surface of a plurality of discrete substrates. 3. An inkjet printing device according to claim 1 or 2, wherein the inkjet printing device comprises at least three rollers arranged to move the substrate relative to the printhead. 4. The inkjet printing apparatus of claim 1 , wherein the rollers are mounted substantially parallel to adjacent rollers such that the angle between the adjacent rollers and the parallel position does not exceed 6 milliradians, and/or wherein , the rollers are rotatable, wherein the inkjet printing device is adapted to control the rotation so that the peripheral speed of adjacent rollers is within 1%. 5. The inkjet printing device of claim 1, wherein the negative gauge pressure acts on the substrate in a region between adjacent rollers. 6. The inkjet printing apparatus of claim 1, wherein the mask member is disposed spaced apart from the substrate. 7. The inkjet printing apparatus of claim 1, wherein the mask member is configured to reduce deformation of the substrate between the rollers. 8. The inkjet printing apparatus according to claim 1, further comprising a guide member for guiding the leading edge of the substrate. 9. The inkjet printing device of claim 1, wherein the substrate comprises a substantially rigid material. 10. The inkjet printing apparatus of claim 1, wherein the substrate is mounted on a deformable surface during printing. 11. The inkjet printing device of claim 1, wherein the inkjet printing device is adapted to move the substrate at a speed greater than 1 m/s. 12. The inkjet printing device of claim 1, wherein the inkjet printing device is adapted to print color images, and/or wherein the inkjet printing device is adapted to print images with a resolution exceeding 120 dpi. 13. A transport apparatus for moving a substrate past a printhead in an inkjet printer, said apparatus comprising: a plurality of rollers configured to move the substrate past the printhead during ink drop attachment to the substrate while the printhead is substantially stationary; as well as a pressure source, wherein the pressure source is configured to apply a negative gauge pressure to the substrate to hold the substrate on the roller in the region of the printhead; a mask element positioned between a first roller and a second roller of the plurality of rollers, thereby defining a first gap between the mask element and the first roller and between the mask element and the first roller. A second gap between the two rollers, wherein the mask element restricts the airflow generated by the vacuum through the first gap and the second gap. 14. A method of printing a substrate in an inkjet printer comprising a plurality of print heads, a plurality of rollers and a pressure source, the method comprising the steps of: moving the substrate on the roller relative to the printhead during printing; and applying a negative gauge pressure to the substrate to hold the substrate on the rollers in the region of the printhead and primarily to throttle the airflow created by the vacuum through the first and second gaps, so The first gap and the second gap are respectively defined by the first roller and the second roller among the plurality of rollers and the mask element arranged therebetween, wherein the first gap is arranged between the mask element and the first roller and the second gap is disposed between the mask element and the second roller; Wherein, the printing head is substantially stationary during the time when the ink droplet is ejected to the substrate.

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