JP7615740B2 - Droplet ejection device and image forming apparatus - Google Patents

Description

The present invention relates to a droplet ejection device and an image forming device.

Conventionally, image forming devices have been known that include a transport unit that transports a medium and an ink ejection unit that ejects ink toward the medium transported by the transport unit. In such image forming devices, if there is variation in the speed at which the medium is transported by the transport unit, the ink ejected from the ink ejection unit may not land at the target position on the medium.

Therefore, in order to ensure that the ink ejected from the ink ejection unit hits the target position on the medium, there is a technique for correcting the ejection timing of the ink ejection unit according to the medium transport speed detected by a sensor (see, for example, Patent Document 1).

One method for correcting the ejection timing according to the transport speed is to calculate the calculated ejection frequency F (Hz) as the ejection timing by substituting the medium transport speed V (mm/sec) and the resolution d (dpi) into the following formula 1.

F=V ×d÷25.4 ...(Formula 1)

The above-mentioned correction method assumes that the ink ejection speed is constant, and changes the calculated ejection frequency according to the transport speed. However, in reality, changing the calculated ejection frequency also causes the ejection speed to fluctuate. For this reason, the above-mentioned correction method cannot be said to sufficiently reduce the error between the target position and the actual landing position.

The present invention has been made to solve these problems, and aims to provide a droplet ejection device that can land droplets at a target position by tracking variations in the transport speed of the medium.

In order to solve the above problem, one aspect of the present invention is a droplet ejection device comprising a transport unit that transports a medium, a droplet ejection unit that ejects droplets onto the medium transported by the transport unit, a transport speed sensor that detects the transport speed of the medium, and a controller that controls the ejection timing of droplets by the droplet ejection unit based on the transport speed detected by the transport speed sensor, wherein the controller comprises a memory that stores a lookup table that holds the ejection frequency that has a smaller landing error of droplets with respect to a target position than the ejection frequency obtained by substituting the transport speed into F = V × d ÷ 25.4 , where V is the transport speed, d is a resolution that is a density of droplets landing on the medium, and F is an ejection frequency that is the ejection timing, and the controller reads out from the lookup table the ejection frequency that corresponds to the transport speed detected by the transport speed sensor, and causes the droplet ejection unit to eject droplets at the read ejection frequency.

According to the present invention, it is possible to land droplets at the target position while following the variation in the transport speed of the medium.

1 is a schematic diagram of an inkjet recording apparatus according to an embodiment of the present invention. FIG. FIG. FIG. 2 is a diagram showing the hardware configuration of the inkjet recording apparatus. FIG. 4 is an explanatory diagram of a lookup table stored in a memory. FIG. 2 is a functional block diagram of the controller. 4 is a flowchart of an image forming process.

Below, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a schematic diagram of an inkjet recording device (image forming device, droplet ejection device) 1 according to an embodiment of the present invention. As shown in FIG. 1, the inkjet recording device 1 includes a paper feed section 10, an image forming section 20, a drying section 30, and a paper ejection section 40. The inkjet recording device 1 forms an image by the image forming section 20 ejecting ink onto paper P (medium) fed from the paper feed section 10. Then, the inkjet recording device 1 dries the ink adhering to the paper P in the drying section 30, and then ejects the paper P to the paper ejection section 40.

The paper feed section 10 includes a paper feed tray 11 on which multiple sheets of paper P are stacked, a feed device 12 that separates and sends out the sheets of paper P one by one from the paper feed tray 11, and a pair of registration rollers 13 that sends the sheets of paper P to the image forming section 20. The feed device 12 can be any known feed device with a feeding function, such as a device using rollers or a device using air suction.

After the leading edge of the paper P sent out from the paper feed tray 11 by the feeding device 12 reaches the pair of registration rollers 13, the pair of registration rollers 13 are driven at a predetermined timing, so that the paper P is fed to the image forming unit 20. Note that in this embodiment, the configuration of the paper feed unit 10 is not particularly limited as long as it has the function of sending out the paper P to the image forming unit 20.

The image forming unit 20 includes a paper transport drum 21 (a rotating member), an ink ejection unit 22 (a droplet ejection unit), a receiving cylinder 24, and a transfer cylinder 25. The paper transport drum 21, the receiving cylinder 24, and the transfer cylinder 25 are examples of a transport unit. The receiving cylinder 24 receives the paper P from the paper feed unit 10. The paper transport drum 21 is a cylindrical rotating member that carries and transports the paper P received by the receiving cylinder 24 on its outer circumferential surface. The ink ejection unit 22 ejects ink toward the paper P transported by the paper transport drum 21. The transfer cylinder 25 transfers the paper P transported by the paper transport drum 21 to the drying unit 30.

The paper P transported from the paper feed unit 10 to the image forming unit 20 has its leading edge gripped by a paper gripper provided on the surface of the receiving drum 24, and is transported as the surface of the receiving drum 24 moves. The paper P transported by the receiving drum 24 is handed over to the paper transport drum 21 at a position opposite the paper transport drum 21.

A paper gripper is also provided on the surface of the paper transport drum 21, and the leading edge of the paper P is gripped by the paper gripper. In addition, multiple suction holes are formed and distributed on the surface of the paper transport drum 21. At each suction hole, a suction air current is generated by the suction device 26 toward the inside of the paper transport drum 21. The paper P is adsorbed to the surface of the paper transport drum 21 by this suction air current. The leading edge of the paper P transferred from the receiving cylinder 24 to the paper transport drum 21 is gripped by the paper gripper, and the paper P is adsorbed to the surface of the paper transport drum 21 by the suction air current, and is transported as the surface of the paper transport drum 21 moves.

The ink ejection unit 22 ejects ink of multiple colors, such as C (cyan), M (magenta), Y (yellow), K (black), white, and gold, to form an image. The ink ejection unit 22 is equipped with ejection heads 23A, 23B, 23C, 23D, 23E, and 23F, each for a different ink color (type of droplet). The ejection heads 23A to 23F eject ink supplied from ink cartridges of each color mounted on the inkjet recording device 1. There are no limitations on the configuration of the ejection heads 23A to 23F, and any configuration can be used as long as they eject liquid.

The ejection operation of the ejection heads 23A to 23F is controlled by a drive signal corresponding to the image data. When the paper P transported by the paper transport drum 21 passes through the area facing the ink ejection section 22, each color ink is ejected from the nozzles provided on the lower surface (nozzle surface) of each ejection head 23A to 23F. As a result, an image corresponding to the image data is formed on the paper P. The ejection heads 23A to 23F can be of a known configuration.

In this embodiment, the image forming unit 20 only needs to have the function of depositing liquid onto the paper P to form an image, and there is no particular limitation on the configuration for ejecting or depositing the liquid.

The drying section 30 includes a drying mechanism section 31 and a suction transport mechanism section 32. The drying mechanism section 31 dries the ink that has adhered to the paper P in the image forming section 20. The suction transport mechanism section 32 transports the paper P transported from the image forming section 20 to the paper discharge section 40. That is, the paper P transported from the image forming section 20 is received by the suction transport mechanism section 32, and then transported by the suction transport mechanism section 32 to pass through the drying mechanism section 31 and delivered to the paper discharge section 40. When passing through the drying mechanism section 31, the ink on the paper P is subjected to a drying process, and the liquid content in the ink, such as water, evaporates, the ink is fixed on the paper P, and curling of the paper P is suppressed.

The paper discharge section 40 is equipped with a paper discharge tray 41 on which multiple sheets of paper P are stacked. The sheets of paper P transported from the drying section 30 by the suction transport mechanism section 32 are stacked and held on the paper discharge tray 41 in sequence. Note that in this embodiment, the paper discharge section 40 only needs to have the function of discharging the sheets of paper P, and its configuration is not particularly limited.

The inkjet recording device 1 of this embodiment is composed of a paper feed unit 10, an image forming unit 20, a drying unit 30, and a paper discharge unit 40, but other functional units may be added as appropriate. For example, the inkjet recording device 1 may include a pre-processing unit between the paper feed unit 10 and the image forming unit 20 that performs pre-processing of the image formation, and a post-processing unit between the drying unit 30 and the paper discharge unit 40 that performs post-processing of the image formation.

Examples of the pre-processing section include a processing liquid application process that applies a processing liquid to the paper P that reacts with the ink to suppress bleeding. However, the contents of the pre-processing are not limited to the above-mentioned examples. Examples of the post-processing section include a paper inversion and transport process that inverts the paper P on which an image has been formed in the image forming section 20 and sends it back to the image forming section 20 to form images on both sides of the paper P, or a process that binds multiple sheets of paper P on which images have been formed. However, the contents of the post-processing are not limited to the above-mentioned examples.

In this embodiment, the inkjet recording device 1 is described as an example of an image forming device. However, the image forming device is not limited to one that visualizes meaningful images such as letters and figures with the ejected droplets, as long as it is equipped with a droplet ejection section that ejects liquid toward the surface of the sheet material to be dried. As another example, the image forming device may be one that forms patterns that have no meaning in themselves.

The medium is not limited to a particular material, and may be any material to which a liquid can be attached, even temporarily, such as paper, thread, fiber, cloth, leather, metal, plastic, glass, wood, or ceramics. The medium may be, for example, a film product, a cloth product for clothing, building materials such as wallpaper or flooring, or leather products. In this embodiment, anything to which a liquid can be attached, even temporarily, is referred to as a "medium." In addition, specific examples of "droplets" are not limited to ink ejected from the ink ejection unit 22, and may also include processing liquid used in a pre-processing unit. In other words, the present invention can be widely applied to droplet ejection devices that cause droplets to land at a target position.

The "ejection head" is a functional component that ejects or sprays liquid from an ejection hole (nozzle). As an energy generation source for ejecting liquid, ejection energy generation means such as a piezoelectric actuator, a thermal actuator that uses an electrothermal conversion element such as a heating resistor, or an electrostatic actuator consisting of a vibration plate and an opposing electrode can be used. However, the ejection energy generation means used is not limited to the above examples.

Next, various sensors equipped in the image forming unit 20 will be described with reference to Figures 2 and 3. Figure 2 is a side view of the main parts of the image forming unit 20. Figure 3 is a plan view of the main parts of the image forming unit 20. Note that the plan views of the ejection heads 23A, 23B, 23C, 23D, 23E, and 23F are the same, so in Figure 3 they are represented by the term "ejection head 23". The image forming unit 20 is equipped with a first encoder 201, a second encoder 211, and a tip position sensor 220.

The first encoder 201 is a rotary encoder composed of an encoder wheel 202 and an encoder sensor 203. The encoder wheel 202 is attached to the shaft 21a of the paper transport drum 21. In addition, N poles and S poles are arranged alternately in the circumferential direction on the outer circumferential surface of the encoder wheel 202. The encoder sensor 203 is disposed in a position facing the outer circumferential surface of the encoder wheel 202.

The encoder wheel 202 rotates together with the paper transport drum 21. The encoder sensor 203 reads the north and south poles arranged on the outer circumferential surface of the encoder wheel 202 and outputs a pulse signal to the controller 100 (see FIG. 4). The first encoder 201 is an example of a sensor that detects the amount of rotation per unit time (i.e., the rotational speed) of the paper transport drum 21.

The second encoder 211 is a linear encoder composed of an encoder scale 212 and a plurality of encoder sensors 213 corresponding to each of the plurality of ejection heads 23A to 23F. The encoder scale 212 is disposed at the axial end of the paper transport drum 21 (outside the area facing the ejection head 23). The encoder scale 212 also extends in the circumferential direction along the outer circumferential surface of the paper transport drum 21. The encoder sensor 213 is disposed at a position facing the encoder scale 212. The encoder sensor 213 is also disposed at the same position as the corresponding ejection head 23 in the circumferential direction of the paper transport drum 21. Furthermore, the encoder sensor 213 is disposed adjacent to the corresponding ejection head 23 in the axial direction of the paper transport drum 21.

The encoder scale 212 rotates together with the paper transport drum 21. The encoder scale 212 has north and south poles arranged alternately in the extension direction. The encoder sensor 213 reads the north and south poles arranged on the encoder scale 212 and outputs a pulse signal to the controller 100. The second encoder 211 is another example of a sensor that detects the amount of rotation per unit time (i.e., the rotational speed) of the paper transport drum 21.

The amount of rotation of the paper transport drum 21 per unit time is correlated with the amount of transport per unit time of the paper P carried and transported by the paper transport drum 21 (i.e., the transport speed). That is, the first encoder 201 and the second encoder 211 are examples of transport speed sensors that detect the transport speed of the paper P. In the image formation process below, a process of detecting the transport speed based on the pulse signal of the second encoder 211 is described, but the transport speed may also be detected based on the pulse signal of the first encoder 201.

The leading edge position sensor 220 is disposed facing the paper transport drum 21, upstream of the ejection heads 23A to 23F in the transport direction of the paper P. The leading edge position sensor 220 detects the position of the leading edge of the paper P carried and transported by the paper transport drum 21, and outputs the detection result to the controller 100.

Figure 4 is a hardware configuration diagram of the inkjet recording device 1. As shown in Figure 4, the inkjet recording device 1 has a configuration in which a CPU (Central Processing Unit) 101, a RAM (Random Access Memory) 102, a ROM (Read Only Memory) 103, a HDD (Hard Disk Drive) 104, and an I/F 105 are connected via a common bus 109.

The CPU 101 is a computing means and controls the operation of the entire inkjet recording device 1. The RAM 102 is a volatile storage medium capable of reading and writing information at high speed, and is used as a working area when the CPU 101 processes information. The ROM 103 is a read-only non-volatile storage medium in which programs such as firmware are stored. The HDD 104 is a non-volatile storage medium with a large storage capacity that allows information to be read and written, and in which the OS (Operating System), various control programs, application programs, etc. are stored.

The inkjet recording device 1 processes a control program stored in ROM 103, an information processing program (application program) loaded from a storage medium such as HDD 104 to RAM 102, and the like, using the calculation function of the CPU 101. This processing constitutes a software control unit including various functional modules of the inkjet recording device 1. The combination of the software control unit thus constituted and the hardware resources mounted on the inkjet recording device 1 constitutes a functional block that realizes the functions of the inkjet recording device 1. That is, the CPU 101, RAM 102, ROM 103, and HDD 104 constitute the controller 100 that controls the operation of the inkjet recording device 1. The RAM 102, ROM 103, and HDD 104 are also examples of memory.

The I/F 105 is an interface that connects the paper feed unit 10, the image forming unit 20, and the drying unit 30 to the common bus 109. That is, the controller 100 controls the paper feed unit 10, the image forming unit 20, and the drying unit 30 through the I/F 105 to execute an image forming process that forms an image indicated by image data on paper P. The image data may be received, for example, from an external device (e.g., a PC) through a communication interface, or may be generated by a scanner mounted on the inkjet recording device 1 reading an original document.

Figure 5 is a diagram for explaining the lookup tables stored in HDD 104. Multiple lookup tables 500A, 500B, 500C, ... are stored in HDD 104. The multiple lookup tables 500A, 500B, 500C, ... (hereinafter, these may be collectively referred to as "lookup table 500") correspond to one of multiple resolutions (e.g., 1200 dpi, 800 dpi, 400 dpi) of the image data recorded on paper P. The resolution is an example of the density of the droplets that land on the medium.

Lookup table 500 includes multiple transport speeds (first column from the left in FIG. 5) and multiple corrected ejection frequencies (second column from the right in FIG. 5) associated with each transport speed. The "transport speed" in FIG. 5 refers to the transport speed of paper P detected by the second encoder 211. The "corrected ejection frequency" in FIG. 5 refers to the ejection timing of ink ejected onto paper P transported at the corresponding transport speed. Meanwhile, the calculated ejection frequency, droplet speed, landing error 1, and landing error 2 in FIG. 5 are items for explaining lookup table 500, and may not be included in lookup table 500.

The ejection frequency indicates the number of ink droplets ejected from a nozzle per unit time. In other words, if the ejection frequency is high, the ink ejection interval will be short (i.e., the ejection timing will be early), and if the ejection frequency is low, the ink ejection interval will be long (i.e., the ejection timing will be late). Below, the lookup table 500A associated with 1200 dpi will be explained in detail.

The calculated ejection frequency (second column from the left in Figure 5) is F (Hz), which is the result of substituting the transport speed V (mm/sec) and resolution d (dpi) into the following formula 1. The calculated ejection frequency F is the ejection timing for landing ink at the target position on the paper P transported at the corresponding transport speed when the droplet speed is fixed (for example, 10.00 m/s). The droplet speed refers to the initial speed of the ink ejected from the nozzle.

F=V ×d÷25.4 ...(Formula 1)

However, in reality, the droplet speed (third column from the left in Figure 5) also fluctuates with changes in the calculated ejection frequency. As a result, when ink is ejected at the corresponding calculated ejection frequency onto paper P transported at the transport speed, a deviation occurs between the target position and the actual landing position, as shown in landing error 1 (third column from the right in Figure 5).

Therefore, during the manufacturing process of the inkjet recording device 1, the developer uses a dedicated device to observe the behavior (e.g., ejection speed, landing error) of the ink ejected from the ejection head 23 while changing the transport speed and ejection frequency. Then, for each combination of multiple transport speeds and multiple resolutions, the developer determines a corrected ejection frequency that reduces the landing error (typically, minimizes the landing error) when ink is ejected at the corresponding calculated ejection frequency.

As a result, when ink is ejected at the corresponding corrected ejection frequency onto paper P transported at each transport speed, the deviation between the target position and the actual landing position, as shown in landing error 2 (first column from the right in Figure 5), becomes smaller than landing error 1.

Figure 6 is a functional block diagram of the controller 100. As shown in Figure 6, the controller 100 includes a conveying speed determination means 601, a lookup table selection means 602, a discharge frequency determination means 603, and a discharge control means 604.

The conveying speed determination means 601 determines the actual conveying speed of the paper P based on the pulse signal output from the second encoder 211. More specifically, the conveying speed determination means 601 determines the conveying speed by dividing the length of the N pole (or S pole) of the encoder scale 212 by the output interval (sec) between two pulse signals that are adjacent in time.

The lookup table selection means 602 selects a lookup table 500 corresponding to the resolution of the image to be formed on the paper P from among the multiple lookup tables 500A, 500B, 500C stored in the HDD 104. The lookup table selection means 602 may obtain the resolution through an external device (e.g., a PC) or an operation panel, for example, or may analyze image data to identify the resolution. The lookup table selection means 602 then reads out the selected lookup table 500 from the HDD 104 and stores it in the RAM 102.

The ejection frequency determination means 603 determines, as the ejection frequency, the corrected ejection frequency corresponding to the conveying speed identified by the conveying speed identification means 601 from among the multiple corrected ejection frequencies in the lookup table 500 stored in the RAM 102. That is, the ejection frequency determination means 603 reads out the corrected ejection frequency corresponding to the current conveying speed from the lookup table 500 selected by the lookup table selection means 602.

The ejection control means 604 causes the ejection head 23 to eject ink droplets at the corrected ejection frequency (ejection timing) determined by the ejection frequency determination means 603. The process of the ejection control means 604 is already well known, so a detailed description will be omitted.

The transport speed identification means 601, the lookup table selection means 602, the ejection frequency determination means 603, and the ejection control means 604 execute the above-mentioned processes in parallel for each of the multiple ejection heads 23A to 23F. In other words, the controller 100 controls the ejection timing of each ejection head 23 based on the transport speed detected by the encoder sensor 213 associated with that ejection head 23.

Next, the image formation process according to this embodiment will be described with reference to FIG. 7. FIG. 7 is a flowchart of the image formation process. The controller 100 starts the image formation process, for example, when an image formation instruction for paper P is obtained from an operator via an operation panel, or when the instruction is received from an external device (e.g., a PC) via a communication network. The image formation instruction includes, for example, image data indicating an image to be formed on paper P and the resolution of the image.

First, the controller 100 starts transporting the paper P received from the paper feed unit 10 via the receiving cylinder 24 by rotating the paper transport drum 21 (S701). Then, the controller 100 waits to execute subsequent processes until the leading edge of the paper P faces the ejection head 23 (S702: No). For example, the controller 100 determines that the leading edge of the paper P faces the ejection head 23 when the number of pulse signals output from the first encoder 201 reaches a predetermined threshold value after the leading edge of the paper P is detected by the leading edge position sensor 220.

Next, after the leading edge of the paper P faces the ejection head 23 (S702: Yes), the controller 100 executes each process of steps S703 to S706. That is, the transport speed specification means 601 specifies the transport speed (S703), and then the lookup table selection means 602 selects the lookup table 500 (S704). Next, the ejection frequency determination means 603 determines the ejection frequency (S705). Next, the ejection control means 604 causes the ejection head 23 to eject ink droplets (S706).

The controller 100 repeatedly executes the processes of steps S703 to S706 each time the second encoder 211 outputs a pulse signal until the formation of the image on the paper P is completed (S707: No). However, when there are no ink droplets to be ejected (for example, when the pixel at the target position is white), the process of step S706 is omitted.

According to the above embodiment, the ejection frequency suitable for each transport speed is stored in advance as a lookup table, and the ejection frequency is determined by referring to the lookup table during image formation. This makes it possible to land ink droplets at the target position while following the variation in the transport speed of the paper P.

Furthermore, according to the above embodiment, a lookup table is prepared for each image resolution. This makes it possible to track variations in the transport speed of the paper P and to land ink droplets at the target position with greater accuracy.

In addition, in FIG. 5, an example is shown in which the transport speed is set at intervals of 0.1 (m/min), but by reducing the intervals between the transport speeds, it is possible to land ink droplets at the target positions with even greater accuracy.

The present invention is not limited to the above-described embodiment, and various modifications are possible without departing from the technical gist of the invention. All technical matters included in the technical ideas described in the claims are covered by the present invention. The above-described embodiment shows a preferred example, but a person skilled in the art can realize various modifications from the disclosed contents. Such modifications are also included in the technical scope described in the claims.

1: Inkjet recording device 10: Paper feed section 11: Paper feed tray 12: Feeding device 13: Pair of registration rollers 20: Image forming section 21: Paper transport drum 21a: Shaft 22: Ink ejection section 23: Ejection head 24: Receiving cylinder 25: Delivery cylinder 26: Suction device 30: Drying section 31: Drying mechanism section 32: Suction transport mechanism section 40: Paper ejection section 41: Paper ejection tray 100: Controller 101: CPU
102: RAM
103: ROM
104: HDD
105: I/F
109: Common bus 201: First encoder 202: Encoder wheels 203, 213: Encoder sensor 211: Second encoder 212: Encoder scale 220: Leading edge position sensor 500: Lookup table 601: Conveying speed specifying means 602: Lookup table selecting means 603: Discharge frequency determining means 604: Discharge control means

JP 2007-237402 A

Claims (5)

A transport unit that transports the medium;
a droplet ejection unit that ejects droplets onto the medium transported by the transport unit;
A conveying speed sensor that detects a conveying speed of the medium;
a controller that controls a timing of droplet ejection by the droplet ejection unit based on the transport speed detected by the transport speed sensor,
The controller:
a memory for storing a lookup table for holding an ejection frequency that has a smaller landing error of droplets with respect to a target position than the ejection frequency obtained by substituting the transport speed into F=V ×d÷25.4 , where V is the transport speed, d is a resolution that is a density of droplets that land on a medium, and F is an ejection frequency that is the ejection timing, and the lookup table is associated with each of a plurality of transport speeds,
The ejection frequency corresponding to the conveying speed detected by the conveying speed sensor is read from the look-up table;
The droplet ejection device is characterized in that the droplet ejection section is caused to eject droplets at the read ejection frequency. the memory stores a plurality of the lookup tables each corresponding to a plurality of the resolutions;
2. The droplet ejection device according to claim 1, wherein the controller reads out the ejection frequency corresponding to the transport speed detected by the transport speed sensor from the lookup table associated with the set resolution. the transport unit includes a rotating member that rotates while supporting the medium on an outer circumferential surface,
the droplet ejection unit is disposed facing an outer circumferential surface of the rotating member,
The conveying speed sensor is
an encoder scale that rotates integrally with the rotating member;
an encoder sensor that reads the encoder scale and outputs a pulse signal;
3. The droplet ejection device according to claim 1, wherein the encoder sensor is disposed at the same position as the droplet ejection section in the circumferential direction of the rotating member. the droplet ejection unit includes a plurality of ejection heads that are disposed at positions spaced apart in a circumferential direction of the rotating member and each ejects a different type of droplet;
the conveying speed sensor includes a plurality of encoder sensors that are disposed at the same positions as the plurality of ejection heads in a circumferential direction of the rotating member,
4. The droplet ejection device according to claim 3, wherein the controller controls the ejection timing of each of the ejection heads based on the pulse signal output from the corresponding encoder sensor. A liquid droplet ejection device according to any one of claims 1 to 4,
The image forming apparatus is characterized in that the droplet ejection section forms an image on a medium by ejecting ink droplets.