A method of applying primer onto a print substrate

Abstract

A method of applying primer (38) to a print substrate (10; 30; 40) by having an ink jet print head (14) ejecting droplets (32) of primer onto the substrate, the method comprising the steps of: • - determining an absorptivity of the print substrate (30), and • - minimizing the amount of primer per area that is necessary for obtaining a continuous primer layer by controlling the size of the primer droplets (32) as a function of the absorptivity such that the droplet size increases with increasing absorptivity.

Description

[0001] A METHOD OF APPLYING PRIMER ONTO A PRINT SUBSTRATE

[0002] BACKGROUND OF THE INVENTION

[0003] 1. Field of the invention

[0004] The invention relates to a method of applying primer onto a print substrate by having an inkjet print head ejecting droplets of primer onto the substrate.

[0005] 2. Description of the Related Art

[0006] In inkjet printing, it is common practice to apply a layer of primer onto the surface of a print substrate before an image is printed with colored ink on top of the primer layer. The purpose of the primer is to improve the quality of the printed image and, in particular, to avoid inter-color bleeding.

[0007] US 2013061767 A1 discloses a method of applying primer wherein the amount of primer per surface area is varied dependent upon the amount of colored ink used for printing the image.

[0008] When an inkjet printer is used for applying the primer, the print resolution, i.e. the number of ink dots per surface area, and the size of the primer droplets ejected onto the substrate must be selected such that the ink drops will coalesce of the surface of the substrate so as to form a continuous primer layer which should preferably have a uniform thickness. However, the tendency of the liquid primer to coalesce will generally depend upon the surface properties of the print substrate, in particular on the absorptivity of the substrate, i.e. the ease with which the liquid primer is absorbed into the interior of the substrate rather than remaining on the surface. In view of this problem, US 2014043393 A1 discloses a method wherein the primer is not applied by means of an ink jet printer but by means of roller coating.

[0009] US 2016303873 A1 discloses another approach in which the absorptivity of the substrate is taken into account by varying the composition of the primer liquid. While a certain amount of primer is necessary in order to achieve a high print quality, a too large amount of primer may compromise the robustness of the printed images, so that, for example, the images may be damaged when the substrate sheets with the freshly printed images thereon are sent through subsequent processing stages in the printer.

[0010] SUMMARY OF THE INVENTION

[0011] It is an object of the invention to provide a method for applying primer that can assure an continuous primer layer while keeping the amount of primer close to minimum.

[0012] In order to achieve this object, the method according to the invention is characterized by the steps of: determining an absorptivity of the print substrate, and minimizing the amount of primer per area that is necessary for obtaining a continuous primer layer by controlling the size of the primer droplets as a function of the absorptivity such that the droplet size increases with increasing absorptivity.

[0013] The amount of primer per surface area can be controlled on the one hand by controlling the print resolution, i.e. the number of primer droplets applied per surface area, and on the other hand by controlling the size of the individual droplets and, therewith, the volume of liquid in a single droplet. It has been found that a continuous primer layer that assures a high print quality can be formed with a minimum amount of primer when the drop size is adapted to the absorptivity of the substrate. When the absorptivity is high, it is preferred to print a relatively small number of relatively large droplets. The liquid of an individual droplet will then form a relatively thick puddle on the surface of the substrate and will tend to spread over a relatively large area and to coalesce with neighboring droplets rather than penetrating in the interior of the substrate. If the same amount of primer is applied in the form of a larger number of smaller droplets, then the droplets will be pinned, i.e. they will be absorbed into the substrate before they have spread enough to coalesce with neighboring droplets. By contrast, if the absorptivity of the substrate is low, then it is preferred to use a relatively large number of relatively small droplets, because the droplets will spread enough to form a uniform primer layer. If the same amount of primer is applied on a low-absorption substrate in the form of a smaller number of larger droplets, then the raster of primer dots becomes so coarse that gaps between the essentially circular dots are likely to occur.

[0014] BRIEF DESCRIPTION OF THE DRAWINGS

[0015] An embodiment example will be now described in conjunction with the drawings, wherein:

[0016] Fig. 1 is a schematic view of an inkjet printer;

[0017] Fig. 2 is a graph showing relations between a necessary amount of primer and a droplet size for two different substrates;

[0018] Fig. 3 shows a distribution of primer dots on the surface of a substrate;

[0019] Fig. 4 illustrates, in a sectional view along the line IV-IV in Fig. 3, the spreading and absorption of small primer droplets on a high-absorptivity substrate;

[0020] Fig. 5 shows the spreading and absorption of fewer but larger droplets on the same high-absorptivity substrate;

[0021] Fig. 6 shows the spreading and absorption of small droplets on a low-absorptivity substrate;

[0022] Fig. 7 shows the spreading and absorption of fewer but larger droplets on the low-absorptivity substrate;

[0023] Fig. 8 is a graph showing a relation between drop size and absorptivity; and

[0024] Fig. 9 is a flow diagram for a method according to the invention. DETAILED DESCRIPTION OF EMBODIMENTS

[0025] An ink jet printer that can be used for practicing the invention has schematically been shown in Fig. 1. Substate sheets 10 are transported on a conveyor 12 in the direction indicated by an arrow and move past a first ink jet print head 14 that is used for applying a primer 16, then past a first curing station 18 for curing the primer, an ink jet print head assembly 20 for applying colored ink so as to form an image 22 on the surface of the primer layer, and a second curing station 24 for curing the image.

[0026] The print head 14 is capable of operating in different print modes in which a smaller or a larger number of inkjet nozzles are active, so that the print resolution is different from print mode to print mode. As is known in the art, each nozzle of the printhead 14 is associated with an actuator, e.g. a piezoelectric actuator, for pressurizing the liquid primer, so that a droplet of primer is expelled from the nozzle. The volume of the droplets that are jetted out can be controlled for example by varying the height of voltage pulses applied to the actuators. In a print mode with a low resolution, the actuators can be controlled to create larger droplets so as to compensate for the smaller number of droplets that hit a given surface area on the substrate 10.

[0027] In order to obtain an image 22 with high quality, without feathering or inter-color bleeding of the colored inks, it is necessary that the primer 16 covers the entire print area on the substrate 10 in the form on a continuous layer with preferably uniform thickness. Consequently, a certain amount of primer is necessary for achieving an acceptable image quality. On the other hand, the amount of primer should be kept as small as possible because an unnecessary thick layer of primer would compromise the robustness of the printed images. The minimum amount of primer that is needed for obtaining an image with acceptable quality depends upon the absorptivity of the surface of the substrate 10 and also on the size of the droplets of primer that are ejected by the print head 14.

[0028] Curves 26 and 28 in Fig. 2 show the relation between the required amount of primer and the drop size for two different substrates with different absorptivities. The curve 26 relates to a low-absorptivity substrate such as Ensocoat 300gsm having an absorptivity score (e.g. amount of absorbed Mg2+) of 17, and the curve 28 relates to a high-absorp- tivity substrate such as Magno Gloss 115gsm having an absorptivity score of 53. In case of the curve 26, the minimum amount of primer that is necessary for preventing the color bleeding increases with increasing dot size, whereas in case of the curve 28, the required amount of primer decreases with increasing dot size. Consequently, in case of a low-absorptivity substrate (curve 26), the amount of primer can be minimized by using a small dot size (corresponding for example to a drop volume of 1.4 pl), and in case of the high-absorptivity substrate (curve 28), the amount of primer can be minimized by using a larger drop size (corresponding for example to a drop volume of 4.1 pl). Of course, using a larger drop size also means using a lower print resolution, so that the number of printed dots per surface area is smaller and, consequently, the total amount of primer, summed over all droplets, remains low.

[0029] Fig. 3 is a plain view of a square aera of a high-absorptivity substrate 30 on which primer dots 32 have been printed with a high print resolution and, correspondingly, with a small drop size. In this example, the absorptivity of the substrate 30 is so large that a major part of the liquid primer has been absorbed into the bulk of the substrate and the primer remaining of the surface has dried so quickly that there was not enough time for the droplets to spread and to coalesce. As a consequence, substantial gaps 34 remain between the neighboring dots, in particular along the diagonal of the matrix of printed dots. Consequently, in order to obtain a continuous primer layer, the volume of the droplets will have to be increased, which results in an increased total amount of primer.

[0030] Fig. 4 is a section taken along the diagonal line I V-l V in Fig. 3 and illustrates the distribution between absorbed primer 36 and primer 38 remaining on the surface in each individual dot 32.

[0031] Fig. 5 illustrates, in a section analogous to Fig. 4, a result that would be obtained if the same amount of primer is applied but with a print resolution half as large, so that the number of dots 32 per surface area is decreased by factor of 4 and, correspondingly, the volume of the droplets is increased by a factor of 4. Due to the larger volume of each droplet, the liquid spreads faster in horizontal direction and the droplets coalesce (i.e. along the diagonal) before a larger amount of primer has been absorbed into the bulk. Consequently, when a larger drop size is used for printing, the amount of primer that is required for obtaining a continuous primer layer is reduced. Conversely, when primer is applied to a low-absorptivity substrate 40, as in Fig. 6, a continuous layer of non-absorbed primer 38 can be obtained by printing with high resolution (with the same resolution as in Fig. 4) and with a small drop size (the same size as in Fig. 4). In this case, the droplets coalesce even along the diagonal because no substantial amount of primer is absorbed into the bulk of the substrate.

[0032] Fig. 7 illustrates the case where the dots 32 have been printed onto the low-absorptivity substrate 40 with reduced print resolution and larger drop size (same as in Fig. 5). In this case, in order for the droplets to coalesce, the liquid has to travel a large distance, in particular along the matrix diagonal, and as the droplet spreads, the depth of the liquid puddle decreases rapidly whereas the surface area of the liquid increases quickly so that the liquid dries before coalescence is completed. In this case, a uniform primer layer could only be obtained by increasing the droplet size even further and, consequently, increasing the amount of primer.

[0033] In Fig. 8 a curve 42 (which is linear in this example) indicates the optimal drop size as function of the absorptivity of the substrate. As has been explained above, the optimum drop size increases with increasing absorptivity.

[0034] It will be understood that each value of the drop size is associated with a corresponding print resolution or number of dots per surface area, so that the print resolution can also be considered as a function of the absorptivity of the substrate.

[0035] The essential steps of a method of applying primer onto a substrate have been illustrated as a flow diagram in Fig. 9.

[0036] In step S1 , the absorptivity of the substrate is determined, for example by reading the absorptivity score from a media catalogue of by measuring the time it takes to absorb a given volume of liquid into the substrate.

[0037] In step S2, an optimal value for the drop size is calculated or read from the curve 42 shown in Fig 8.

[0038] Then, in step S3, the print resolution that is associated with the drop size found in step

[0039] 2 is selected for printing. Of course, when a printhead with a given array of print nozzles is used for printing, the print resolution can only be changed in relatively coarse steps (e.g. 1200 dpi, 600 dpi, 300 dpi, ...). Thus, selecting the print resolution means selecting a print resolution that is available for the printhead and comes closest to the print resolution that is linked with the optimal drop size.

[0040] Then, however, if, for example, the available print resolution is smaller than the one that would really be appropriate for the given drop size, the amount of primer may be become too small. For this reason, the drop size is “fine-tuned” in a subsequent step S4 in order to actually achieve a total amount of primer that, given the restricted choice of print resolutions, is the smallest amount that can assure a continuous primer layer.

[0041] Then, in step S5, the primer layer is printed with the fine-tuned drop size and the se- lected print resolution.

Claims

Claims1. A method of applying primer (16) onto a print substrate (10; 30; 40) by having an inkjet print head (14) ejecting droplets (32) of primer onto the substrate, the method comprising the steps of: determining an absorptivity of the print substrate (10; 30; 40), and minimizing the amount of primer per area that is necessary for obtaining a continuous primer layer by controlling the size of the primer droplets (32) as a function of the absorptivity such that the droplet size increases with increasing absorptivity.

2. The method according to claim 1, wherein the size of the primer droplets (32) is controlled by controlling actuators of the print head (14).

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