Printing correction method for external drum platemaking machine

Abstract

The present invention relates to a printing correction method for an external drum platemaking machine, belonging to the technical field of CTP printing platemaking, which comprises the following steps: installing a distance sensor on the optical print head of the external drum printing device; measuring the initial distance D0 from the optical print head to the printing working surface of the plate material by the distance sensor before printing; measuring the real-time distance D0 from the optical print head to the printing working surface of the plate material by the distance sensor during printing. θ , based on the initial distance D0 from the optical print head to the plate printing surface and the real-time distance D θ Calculate and adjust the real-time rotational angular velocity ω of the drum in an external drum printing device θ , the calculation formula is: θ =ω0R0 / R θ , where R0=L-D0, R θ =L-D θ The present invention maintains a constant linear velocity on the printing surface of the plate by modifying the angular velocity value of the compensation drum, thereby correcting deformation errors of the printed image caused by various reasons.

Description

Technical Field

[0001] The invention belongs to the technical field of CTP printing platemaking, and in particular relates to a printing correction method for an external drum type platemaking machine. Background Art

[0002] Computer-to-plate machines, also commonly referred to as CTP platesetters, are prepress equipment and are categorized into four main types: internal drum, external drum, flatbed, and curved. Of these four types, internal drum and external drum are the most commonly used; high-end CTP platesetters with superior performance all use external drums.

[0003] In the external drum platemaking machine system, the printing plate is fixed on the outside of the drum. When the drum rotates in the circumferential direction at a speed of several hundred revolutions per minute, the plate rotates at the same speed as the drum. At the same time, the laser irradiates the printing plate to complete the scanning of the printing plate. Generally, in order to improve production efficiency, multiple laser beams are often used for scanning. The core printing component of the external drum platemaking machine consists of a rotating drum 1, a horizontal linear motion guide rail 2, a high-precision optical print head 3, and a slow scanning motor for driving the optical print head 3 to perform linear motion on the guide rail 2, such as Figure 1 shown.

[0004] During printing, the plate material 4 to be printed is wrapped around the drum 1, and the drum 1 rotates. The optical print head 3 is installed on the guide rail 2 and moves horizontally. The laser in the optical print head 3 is turned on and off according to the pattern to be printed. The horizontal movement of the optical print head 3 is combined with the rotation of the drum 1 to scan and print the completed pattern on the surface of the plate material 4. Figure 2 As shown, when the drum is uniform and the plate to be printed has a uniform thickness, the printed pattern on the plate will not deform in size. The key is that the linear velocity of the plate surface is constant. If the linear velocity varies, the printed pattern will also deform. Uneven drum rotation due to machining errors, uneven plate thickness due to excessive errors, or the plate not being completely in contact with the drum due to varying tightness when wrapped around the drum can all cause the surface of the printed plate to become uneven, causing the equivalent rotation radius R to vary. Since the angular velocity of the drum motor is constant, the linear velocity of the plate surface will vary randomly, resulting in various random deformations of the printed image, resulting in inaccurate dimensions and unqualified prints.

[0005] The rotating drum can range in diameter from 100mm to 2000mm and in length from 300mm to 3000mm, depending on the print format. The sheet material to be printed is wrapped around the drum, and the laser head prints on it as it rotates. The commonly used precision in printing is 2400 DPI or higher, meaning that a single pixel is approximately 10µm. This places very high demands on the platesetter's optical system for image quality. High-precision optical systems also have a very short available beam depth of field, and focus drift exceeding 20µm can severely impact image quality. Therefore, external drum platesetters require extremely high surface accuracy for the drum. Conventional platemaking equipment requires a total runout of no more than 10µm while the drum rotates. Furthermore, the horizontally moving slow scan axis must maintain an error of no more than 1–2µm across the entire print format. This precision requirement makes the manufacturing process of this equipment very challenging.

[0006] In traditional CTP offset printing, printing resolution is typically 2400 or 2540 DPI. As the printing industry gradually becomes saturated, new markets are placing new demands on printing. Platesetters used in the circuit board industry, also known as photoplotters, require print resolutions between 9000 and 51200 DPI. Flexographic platesetters used in flexographic printing also require resolutions above 5080 DPI. Furthermore, flexographic platesetters print on larger formats, with plate thicknesses ranging from 0.9 to 4 mm, exceeding the 0.27 mm required by traditional offset printing. These printing requirements necessitate higher optical resolution, a smaller depth of field, and even higher requirements for the drum's precision. These larger print formats and higher surface runout requirements mean that the mechanical precision of the drum is no longer sufficient to meet the printing requirements of these new applications.

[0007] Therefore, in areas such as photoplotter printing, before printing a final document, a test document must be printed and the printed pattern accurately measured. This determines the actual dimensional error values ​​of the entire device at different locations across the print format. These values ​​are then input into the file generation system, which then compensates for data pattern distortion in the document being printed, ensuring that the desired print pattern is produced. This pattern correction process involves first measuring the test pattern error; then generating an error file based on the measurement results, which is used for image correction; and finally, rasterizing and screening the image to produce a print file with only two color channels, each with a specific resolution. This correction method presents the following issues: 1. Most platemaking centers receive customer files already screened, making it impossible to perform pattern correction on these files, as this would result in significant moiré patterns or a significant loss of resolution. 2. High-resolution files are extremely data-intensive, making distortion correction extremely time-consuming. For example, a standard 3-square-meter printing plate with a 5080 DPI resolution contains 120 billion pixels. At a high resolution of 25,400 DPI, the number of pixels reaches 30 trillion pixels. This enormous amount of data requires significant time to correct for data distortion on each plate. Third, the average thickness of flexographic plates is 2mm. Even if the flexographic platemaking machine's drum machining precision meets printing requirements, the thickness of each plate is not uniform. Furthermore, plates of this thickness are primarily made of elastic material, which, compared to the aluminum plates used in traditional offset printing, is not only heavier, resulting in significant thickness variations. Furthermore, during rotation, centrifugal force creates a gap between the plate and the drum. The tension applied to each plate varies, creating a different gap between the plate and the drum. Consequently, the surface error of each plate varies during printing, making it impossible to measure and correct the drum error and apply this fixed error calibration file to each printed image, as is done with traditional methods. Summary of the Invention

[0008] The present invention provides a printing correction method for an external drum plate-making machine, so as to solve the problems of various random deformations, inaccurate dimensions and the like of an image caused by uneven surface of a plate to be printed during the printing process of the external drum plate-making machine.

[0009] In order to solve the above technical problems, the technical solution provided by the present invention is:

[0010] The present invention relates to a printing correction method for an external drum plate-making machine, which comprises the following steps: installing a distance sensor on an optical print head of an external drum printing device; measuring an initial distance D0 from the optical print head to a printing work surface of a plate material by using the distance sensor before printing; and measuring a real-time distance D0 from the optical print head to a printing work surface of a plate material by using the distance sensor during printing. θ, based on the initial distance D0 from the optical print head to the plate printing surface and the real-time distance D θ Calculate and adjust the real-time rotational angular velocity ω of the drum in an external drum printing device θ , the calculation formula of the real-time rotation angular velocity of the drum is:

[0011] ω θ =ω0 R0 / R θ (1),

[0012] Among them, ω0 is the initial rotation angular velocity of the drum, R0 is the vertical distance from the printing coordinate point on the printing working surface of the plate to the axis of the drum in the initial state, and R θ is the real-time vertical distance from the printing coordinate point on the plate printing work surface to the axis of the drum during printing, and θ is the angle of rotation of the drum;

[0013] R0=L- D0 (2),

[0014] R θ =L-D θ (3),

[0015] Wherein, L is the distance from the optical print head to the axis of the drum.

[0016] Preferably, a distance sensor is used to measure the real-time distance D between the optical print head and the printing surface of the plate during printing. θ When , the sampling interval of the distance measuring sensor in the horizontal and vertical directions of the moving direction of the optical print head is H.

[0017] Preferably, when determining the sampling interval H of the distance measuring sensor in the horizontal and vertical directions of the moving direction of the optical print head, the distance difference ΔD between the optical print head and the printing working surface of the plate measured at the two preceding and following sampling points satisfies the following formula:

[0018] △D≤25400 / 2πP (4),

[0019] Where P is the DPI value of the printing resolution, π is the pi, and △D is in μm.

[0020] The coefficient 25400 in this formula is related to the length unit. If the unit of △D is mm, then Formula 4 becomes △D≤25.4 / 2πP.

[0021] Preferably, the distance measuring sensor and the optical print head are spaced apart, and the spacing is equal to a sampling interval H of the distance measuring sensor in the horizontal and vertical directions of the moving direction of the optical print head.

[0022] Preferably, the optical print head moves at a constant speed during the printing process.

[0023] Preferably, the distance measuring sensor is installed on the side of the optical print head facing the rotating drum.

[0024] Preferably, the distance measuring sensor is a laser triangulation displacement distance measuring sensor.

[0025] Compared with the prior art, the technical solution provided by the present invention has the following beneficial effects:

[0026] 1. The present invention relates to a method for correcting printing of an external drum plate-making machine. Based on a conventional external drum plate-making machine, a distance sensor is added to the optical print head. Before printing, the distance sensor is used to measure the initial distance D0 from the optical print head to the printing surface of the plate; during printing, the distance sensor is used to measure the real-time distance D from the optical print head to the printing surface of the plate. θ , based on the initial distance D0 and the real-time distance D θ Calculate and adjust the real-time rotational angular velocity ω of the drum in an external drum printing device θ , modify the angular velocity of the compensation drum. By modifying the angular velocity value of the compensation drum, the linear velocity of the printing surface of the plate is always kept constant, thereby correcting the deformation error of the printed image caused by various reasons. This correction method does not require correction of the data at the trillion-pixel level of the print file itself, and the computing pressure of the software and hardware is greatly reduced.

[0027] 2. The present invention relates to a method for correcting printing on an external drum plate-making machine. Based on a conventional external drum plate-making machine, a distance sensor is added to the optical print head. The distance sensor is spaced apart from the optical print head by a distance equal to the sampling interval H of the distance sensor in the horizontal and vertical directions of the optical print head's movement. The distance sensor can lead the optical print head by a distance H in the horizontal direction of movement, so that when the optical print head prints the data of a circle, the distance sensor is already measuring the next circle's D. θ When the optical print head prints the next circle, the current D θ Value, for the drum angular velocity ω θ Just make compensation and save the data in advance to avoid delays in calculating real-time data. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 This is a schematic diagram of the structure of the core printing components of an existing external drum platemaking machine;

[0029] Figure 2 This is the printing principle diagram of the existing external drum platemaking machine;

[0030] Figure 3 This is a schematic diagram of a printing correction method for an external drum platemaking machine according to the present invention;

[0031] Figure 4 This is a sampling point distribution diagram of a distance measuring sensor during printing of an external drum type platemaking machine according to the present invention;

[0032] Figure 5 This is a schematic diagram of the positional relationship between the distance sensor and the optical print head.

[0033] Reference numerals: 1-rotating drum, 2-guide rail, 3-optical printing head, 4-plate, 5-distance sensor. DETAILED DESCRIPTION

[0034] In order to further understand the content of the present invention, the present invention is described in detail with reference to the examples. The following examples are used to illustrate the present invention but are not used to limit the scope of the present invention.

[0035] Refer to the attached Figure 3 As shown, the processing error of the drum 1, the thickness error of the plate 4, the difference in the tightness of the plate 4 wrapped around the drum 1, etc., can all cause the surface of the plate to be printed to become uneven, causing the equivalent rotation radius R to change. When the angular velocity of the motor of the drum 1 and the speed V of the horizontal scanning of the optical print head 3 are both constant values, the linear velocity of the plate surface will change randomly, and the printed image will have various random deformations, resulting in inaccurate dimensions and unqualified printed products. Based on the above reasons, the external drum platemaking machine of the present invention is used in such Figure 1 On the basis of the conventional external drum platemaking machine shown, a distance sensor 5 is additionally provided, and based on the improved external drum platemaking machine, the printing method of the external drum platemaking machine is corrected.

[0036] The present invention relates to a printing correction method for an external drum plate-making machine, which comprises the following steps:

[0037] A distance sensor 5 is installed on the optical print head of the external drum printing device. The distance sensor 5 can be in any form, such as a laser triangulation distance sensor, which is installed on the side of the optical print head 3 facing the drum.

[0038] Before printing, the distance sensor 5 is used to measure the initial distance D0 from the optical print head 3 to the printing surface of the plate 4. The initial distance D0 from the optical print head 3 to the printing surface of the plate 4 is the sum of the distance from the measuring point of the distance sensor 3 to the plate 4 and the distance that the distance sensor 3 protrudes from the optical print head 3. However, compared with the distance from the measuring point of the distance sensor 3 to the plate 4, the distance that the distance sensor 3 protrudes from the optical print head 3 is negligible. Therefore, the distance from the measuring point of the distance sensor 3 to the plate 4 can be equated with the initial distance D0 from the optical print head 3 to the printing surface of the plate 4. Based on this, the vertical distance R0 from the printing coordinate point on the printing surface of the plate to the axis of the drum in the initial state is calculated. The calculation formula is:

[0039] R0=L- D0 (2),

[0040] Wherein, L is the distance from the optical print head 3 to the axis of the drum 1. Although the surface of the drum 1 may have processing errors, the bearings for the rotation of the drum 1 are fixed on the base. Therefore, the distance L from the optical print head 3 to the axis of the drum 1 remains unchanged during the printing process.

[0041] During the printing process, the optical print head moves at a constant speed. The method for determining the moving speed of the optical print head belongs to the conventional technology in this field, and the present invention is applicable regardless of the moving speed of the optical print head. Therefore, the present invention will not be described in detail. At the same time, a distance sensor is used to measure the real-time distance D from the optical print head to the printing working surface of the plate. θ In this process, the whole plate 4 is measured according to the actual D θ The error order of magnitude is used to determine the specific number of sampling distance values ​​per circle, as well as the distance the optical print head moves in the horizontal direction to sample a circle of data. Considering that the surface of the plate 4 is isotropic in the direction of rotation of the drum and the horizontal movement direction of the optical print head, the sampling distance D in the rotation direction is θ The position interval of is the same as the horizontal motion sampling interval, that is, the sampling interval of the ranging sensor 5 in the horizontal and vertical directions of the moving direction of the optical print head 3 is H. Figure 4 As shown; the distance difference △D from the optical print head to the printing surface of the plate measured at the two sampling points before and after satisfies the following formula:

[0042] △D≤25400 / 2πP (4),

[0043] Where P is the DPI value of the print resolution. DPI is a commonly used resolution unit in printing, meaning the number of pixels per inch. 1inch = 25.4mm = 25400um. The 25400 in the formula is the value of 25400um, representing 1 inch. π is the ratio of pi, and the unit of △D is um.

[0044] Taking specific data as an example, if P = 2400 DPI, when △D = 25400 / 2π2, the △D error will cause the actual image resolution to be 2399 DPI or 2401 DPI, depending on whether the △D value is positive or negative. Therefore, when △D ≤ 25400 / 2πP, the resolution error caused by △D is less than 1, that is, the image resolution at this time can still be considered to be around 2400 DPI.

[0045] Based on the measured real-time distance D between the optical print head 3 and the printing surface of the plate 4 θ , calculate the real-time vertical distance R from the printing coordinate point on the printing working surface of plate 4 to the axis of drum 1 during printing θ , the calculation formula is:

[0046] R θ =L-D θ (3).

[0047] When the drum is uniform and the thickness of the plate to be printed is uniform, the relationship between the scanning linear velocity V and the angular velocity ω of the drum motor is V=ωR. However, according to the above steps, it can be seen that in actual situations, the thickness of the drum and / or the plate is not uniform. Therefore, it is necessary to calculate and adjust the real-time rotation angular velocity ω of the drum 1 in the external drum printing device. θ , the calculation formula of the real-time rotation angular velocity of the drum 1 is:

[0048] ω θ =ω0 R0 / R θ (1);

[0049] At this time, during the entire printing process, the laser emitted by the optical print head 3 moves uniformly at a scanning linear velocity V on the surface of the plate 4, and the linear velocity of the printing working surface of the plate 4 can be adjusted to remain consistent.

[0050] Preferably, the distance sensor 5 is spaced apart from the optical print head 3, and the spacing is equal to the sampling interval H of the distance sensor 5 in the horizontal and vertical directions of the moving direction of the optical print head 3, as shown in FIG. Figure 5 As shown, the distance sensor can be ahead of the optical print head 3 in the horizontal moving direction by a distance H, so that when the optical print head 3 prints the data of this circle, the distance sensor 5 is already measuring the distance D of the next circle. θ Data and save it; when the optical print head 3 prints to the next circle, the current D θ Value, for the drum angular velocity ω θ Just make compensation and save the data in advance to avoid delays in calculating real-time data.

[0051] The present invention has been described in detail above with reference to the embodiments. However, the contents described are only preferred embodiments of the present invention and should not be considered as limiting the scope of the present invention. All equivalent changes and improvements made within the scope of the present invention should still fall within the scope of the patent coverage of the present invention.

Claims

1. A printing correction method for an external drum platemaking machine, characterized in that: It includes the following steps: Installing a distance sensor on the optical print head of the external drum printing device; Before printing, use the distance sensor to measure the initial distance from the optical print head to the printing surface of the plate D 0; During printing, the distance sensor is used to measure the real-time distance from the optical print head to the printing surface of the plate. D θ The sampling interval of the distance measuring sensor in the horizontal and vertical directions of the optical print head moving direction is H. When determining the sampling interval H of the distance measuring sensor in the horizontal and vertical directions of the optical print head moving direction, the difference in the distance from the optical print head to the printing working surface measured at the two sampling points before and after is △ D All satisfy the following formula: △ D ≤25400 / 2 πP （4）， in, P The DPI value of the printing resolution. π is pi; Based on the initial distance from the optical print head to the plate printing surface D 0 and the real-time distance from the optical print head to the plate printing surface D θ Calculate and adjust the real-time angular velocity of the drum in an external drum printing device ω θ , the calculation formula of the real-time rotation angular velocity of the drum is: ω θ = ω 0 R 0 / R θ （1）， in, ω 0 is the initial rotation angular velocity of the drum, R 0 is the vertical distance from the printing coordinate point on the plate printing surface to the drum axis in the initial state. R θ is the real-time vertical distance from the printing coordinate point on the plate printing work surface to the axis of the drum during printing, and θ is the angle of rotation of the drum; R 0= L - D 0（2）， R θ = L - D θ （3）， in, L It is the distance from the optical print head to the axis of the drum.

2. The printing correction method of the external drum platemaking machine according to claim 1, characterized in that: The distance measuring sensor and the optical print head are spaced apart, and the spacing is equal to the sampling interval H of the distance measuring sensor in the horizontal and vertical directions of the moving direction of the optical print head.

3. The printing correction method of the external drum platemaking machine according to claim 1, characterized in that: The optical printing head moves at a constant speed during the printing process.

4. The printing correction method of the external drum platemaking machine according to claim 1, characterized in that: The distance measuring sensor is installed on the side of the optical print head facing the drum.

5. The printing correction method of the external drum platemaking machine according to claim 1, characterized in that: The distance measuring sensor adopts a laser triangulation displacement distance measuring sensor.

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