How to understand the flexographic version of the overall platemaking At present, the domestic introduction of flexible plates is basically a solid photosensitive resin plate, the thickness of the plate is selected according to the requirements of the flexographic printing press, the common plate thickness is 1.70 mm and 2.29 mm. The flexographic plate making process is the process of transferring the graphic to the plate through the negative graphic film, which mainly includes the following steps:
(1) Back exposure of the plate material with ultraviolet light, the role of which is to determine the height of relief on the plate, ie the depth of corrosion, and solidify the substrate.
(2) Expose the printing plate together with the film under the influence of ultraviolet light to form a graphic printing portion on the printing plate and cure it.
(3) The printing plate is scrubbed in a solvent to remove the unexposed portions of the plate material so that the graphic portion is embossed.
(4) The printing plate is dried in a dryer to promote the volatilization of the solvent absorbed in the plate, so that the thickness of the plate is restored to the original standard value.
(5) The post-exposure and de-tack treatment of the dried plates can further solidify the shoulders and bases and improve the flexographic printing performance. The high-quality flexographic printing plate should have a printing rate of more than 500,000 prints and even as much as 1 million prints.
How to calculate the shrinkage of a flexographic film In the previous question and answer, it has been pointed out that the shrinkage is only related to the diameter of the plate cylinder (including double-sided tape and plate) and the thickness of the plate itself. Here is a formula used to calculate the shrinkage (percentage) of a film when it is used in a plane exposure to make a flexographic plate: Reduced volume (percent) = K/Rx100%.
In the formula, the coefficient K depends on the thickness of the plate used. When the thickness is 1.70 mm, K = 9.89 mm; and when the thickness is 2.29 mm, K = 13.56 mm. R is the print length of the plate cylinder in millimeters. For example, if the print length of a certain product is R=235mm, the thickness of the plate is 1.70mm, K=9.89mm, then the shrinkage quantity is 9.89/235x100%=4.21%.
When designing an ink draft, the film should be longitudinally reduced according to the above proportions, and then plate-exposure. This will enable the printed product to meet the dimensional requirements. Further explanation is that when the actual production is analyzed and estimated, the length is printed. The determination of R must take into account various factors. For example, when the module of the roller gear is 1/8 inch, the printing length can only be increased or decreased in increments of 3.175 mm; Other factors can also reduce the amount of paper consumed, leaving a sticky version. Weighing, and the limitations of machining indentation die-cutting tools, etc.
How to do back exposure test
We can first determine the back exposure time for different etching depth by doing back exposure test on the plate making machine. For comparison, we use multiple exposures with different exposure times for the same plate. Take a piece of media about 320 mm long and 100 mm wide. When performing back exposure on a platemaking machine, we use red cover strips to partially cover the plate, and the platesetter makes multiple exposures, resulting in five different back exposure times. Regional plate.
In addition, about 20 mm of blank space is left at the end of the plate for the plate washer. Suppose we choose a flexographic plate with a thickness of 1.70 mm produced by the Dutch AV-500 combined platemaking machine and the GRACE company. After partial coverage and multiple exposures, we have exposure times of 0 on the same plate. The five parts of 15, 20, 25, and 30 seconds were exposed and placed on the drum of the printing plate for washing, about 7 minutes. It is then dried in a drying oven at a temperature of 50°C for 1.5 hours. Finally, the printing plate is taken out, and four different plate thickness values ​​on the printing plate can be measured by using a caliper, while the unexposed portion of the printing plate remains only the hard polyester plate base.
Through the above test, the platemaking machine can be quickly familiar with, and its test results also have guiding significance for adjusting the corrosion depth of the printing plate. From the above experiments, it can be seen that the length of the back exposure time will affect the thickness of the substrate. Therefore, different back-exposure times should be used for plate making for different printing images. Still using a 1.70 mm thick flexographic plate as an example, the anti-white and large-size solids on the plate will have a deeper etching depth (ie, a thinner base plate) up to 1.0 mm, and the back exposure time will be the shortest. The general picture and text part has a medium depth of corrosion, about 0.85 mm, and the back exposure time is moderate. As for the fine lines and dots, the corrosion depth should be the shallowest, about 0.63 mm, so the back exposure time should be the longest. .
In addition, there are two other points worth noting: When the printer changes to a flexible plate of another thickness or another brand of flexographic plate, the back exposure test must be performed again. With the extension of UV lamp usage time, its light intensity will be reduced. At this time, the back exposure time should be adjusted accordingly.
How to do positive exposure test
The frontal exposure is also called the main exposure, which forms the graphic printing part of the flexo version. Proper exposure and back exposure work together to produce high-quality printing plates that meet the requirements of various graphic types. In general, underexposure will cause the bottom of the shoulder around the graphic to be washed away, which will affect the life of the printing plate. Excessive positive exposure will result in insufficient corrosion, which can easily cause stencil printing and cause waste during printing.
Before conducting the front exposure test, we recommend that the operator prepare a test “special sample†dedicated to the production of flexible plates. The “samples†should include small anti-white, normal anti-white and field, common graphic, and small The basic printing and graphic types of lines and characters, dots, etc., and reflecting a certain number of lines (eg, 150 lpi) from 10% to 100% on the ground are all reflected in “samplesâ€.
The positive exposure test can be performed using a "swatch" on the platesetter with partial coverage and multiple exposures. In the early stage, it is also possible to start from the less difficult business, from easy to difficult, to accumulate experience, and to fully master the technology of making flexible versions. In theory, the longer the positive exposure time required for finer graphics, the more important it is to remember the combined effect of back exposure. If the anti-white characters need less back exposure and positive exposure, and the fine dots need a shallower depth of corrosion, their back exposure and positive exposure time will be longer.
Through common time exploration and practice, platemaking operators need to understand the limits of their own plate making, such as whether the finest lines, the smallest characters, the densest dots, and 3% of the dots can be “stoppedâ€, and whether 50% of the dots are highlighted. "Open up" and so on.
What should be noticed when washing and drying after washing plate
With the increase in the number of solvents used, the time required for brushing the plate should be appropriately extended; according to the different plates, the position between the fine brush and the plate drum should be adjusted so as to adjust the pressure of the fine brush and the surface of the plate; if the system is extremely thin The outlets should be based on the standard thickness position to further reduce the pressure of the fine brush and prevent the outlets from being washed away.
Drying temperature is about 50 °C, time is about 1.5 to 2 hours; should be removed several times in the drying process, check the surface of the layout with or without solvent spots, can use fiber clean cloth, dip Some fresh solvent, gently wipe the surface of the plate, and then put it back in the drying oven to continue drying.
Post-exposure and de-sticking media can greatly improve printability. The prepared flexible plate is best placed for 12 hours and then subjected to an offset printing so that all the residual solvent absorbed into the plate at the time of plate making can be volatilized.
In short, in actual operation, the plate-making staff of the flexographic plates must closely cooperate with the personnel of the departments of ink draft design, film production, and printing, communicate with each other, and sum up lessons learned in a timely manner so as to improve the overall level of flexographic printing. (Liang Xinyu Witlon (Hong Kong) Limited) ("The Graphic Arts Society Monthly")
(1) Back exposure of the plate material with ultraviolet light, the role of which is to determine the height of relief on the plate, ie the depth of corrosion, and solidify the substrate.
(2) Expose the printing plate together with the film under the influence of ultraviolet light to form a graphic printing portion on the printing plate and cure it.
(3) The printing plate is scrubbed in a solvent to remove the unexposed portions of the plate material so that the graphic portion is embossed.
(4) The printing plate is dried in a dryer to promote the volatilization of the solvent absorbed in the plate, so that the thickness of the plate is restored to the original standard value.
(5) The post-exposure and de-tack treatment of the dried plates can further solidify the shoulders and bases and improve the flexographic printing performance. The high-quality flexographic printing plate should have a printing rate of more than 500,000 prints and even as much as 1 million prints.
How to calculate the shrinkage of a flexographic film In the previous question and answer, it has been pointed out that the shrinkage is only related to the diameter of the plate cylinder (including double-sided tape and plate) and the thickness of the plate itself. Here is a formula used to calculate the shrinkage (percentage) of a film when it is used in a plane exposure to make a flexographic plate: Reduced volume (percent) = K/Rx100%.
In the formula, the coefficient K depends on the thickness of the plate used. When the thickness is 1.70 mm, K = 9.89 mm; and when the thickness is 2.29 mm, K = 13.56 mm. R is the print length of the plate cylinder in millimeters. For example, if the print length of a certain product is R=235mm, the thickness of the plate is 1.70mm, K=9.89mm, then the shrinkage quantity is 9.89/235x100%=4.21%.
When designing an ink draft, the film should be longitudinally reduced according to the above proportions, and then plate-exposure. This will enable the printed product to meet the dimensional requirements. Further explanation is that when the actual production is analyzed and estimated, the length is printed. The determination of R must take into account various factors. For example, when the module of the roller gear is 1/8 inch, the printing length can only be increased or decreased in increments of 3.175 mm; Other factors can also reduce the amount of paper consumed, leaving a sticky version. Weighing, and the limitations of machining indentation die-cutting tools, etc.
How to do back exposure test
We can first determine the back exposure time for different etching depth by doing back exposure test on the plate making machine. For comparison, we use multiple exposures with different exposure times for the same plate. Take a piece of media about 320 mm long and 100 mm wide. When performing back exposure on a platemaking machine, we use red cover strips to partially cover the plate, and the platesetter makes multiple exposures, resulting in five different back exposure times. Regional plate.
In addition, about 20 mm of blank space is left at the end of the plate for the plate washer. Suppose we choose a flexographic plate with a thickness of 1.70 mm produced by the Dutch AV-500 combined platemaking machine and the GRACE company. After partial coverage and multiple exposures, we have exposure times of 0 on the same plate. The five parts of 15, 20, 25, and 30 seconds were exposed and placed on the drum of the printing plate for washing, about 7 minutes. It is then dried in a drying oven at a temperature of 50°C for 1.5 hours. Finally, the printing plate is taken out, and four different plate thickness values ​​on the printing plate can be measured by using a caliper, while the unexposed portion of the printing plate remains only the hard polyester plate base.
Through the above test, the platemaking machine can be quickly familiar with, and its test results also have guiding significance for adjusting the corrosion depth of the printing plate. From the above experiments, it can be seen that the length of the back exposure time will affect the thickness of the substrate. Therefore, different back-exposure times should be used for plate making for different printing images. Still using a 1.70 mm thick flexographic plate as an example, the anti-white and large-size solids on the plate will have a deeper etching depth (ie, a thinner base plate) up to 1.0 mm, and the back exposure time will be the shortest. The general picture and text part has a medium depth of corrosion, about 0.85 mm, and the back exposure time is moderate. As for the fine lines and dots, the corrosion depth should be the shallowest, about 0.63 mm, so the back exposure time should be the longest. .
In addition, there are two other points worth noting: When the printer changes to a flexible plate of another thickness or another brand of flexographic plate, the back exposure test must be performed again. With the extension of UV lamp usage time, its light intensity will be reduced. At this time, the back exposure time should be adjusted accordingly.
How to do positive exposure test
The frontal exposure is also called the main exposure, which forms the graphic printing part of the flexo version. Proper exposure and back exposure work together to produce high-quality printing plates that meet the requirements of various graphic types. In general, underexposure will cause the bottom of the shoulder around the graphic to be washed away, which will affect the life of the printing plate. Excessive positive exposure will result in insufficient corrosion, which can easily cause stencil printing and cause waste during printing.
Before conducting the front exposure test, we recommend that the operator prepare a test “special sample†dedicated to the production of flexible plates. The “samples†should include small anti-white, normal anti-white and field, common graphic, and small The basic printing and graphic types of lines and characters, dots, etc., and reflecting a certain number of lines (eg, 150 lpi) from 10% to 100% on the ground are all reflected in “samplesâ€.
The positive exposure test can be performed using a "swatch" on the platesetter with partial coverage and multiple exposures. In the early stage, it is also possible to start from the less difficult business, from easy to difficult, to accumulate experience, and to fully master the technology of making flexible versions. In theory, the longer the positive exposure time required for finer graphics, the more important it is to remember the combined effect of back exposure. If the anti-white characters need less back exposure and positive exposure, and the fine dots need a shallower depth of corrosion, their back exposure and positive exposure time will be longer.
Through common time exploration and practice, platemaking operators need to understand the limits of their own plate making, such as whether the finest lines, the smallest characters, the densest dots, and 3% of the dots can be “stoppedâ€, and whether 50% of the dots are highlighted. "Open up" and so on.
What should be noticed when washing and drying after washing plate
With the increase in the number of solvents used, the time required for brushing the plate should be appropriately extended; according to the different plates, the position between the fine brush and the plate drum should be adjusted so as to adjust the pressure of the fine brush and the surface of the plate; if the system is extremely thin The outlets should be based on the standard thickness position to further reduce the pressure of the fine brush and prevent the outlets from being washed away.
Drying temperature is about 50 °C, time is about 1.5 to 2 hours; should be removed several times in the drying process, check the surface of the layout with or without solvent spots, can use fiber clean cloth, dip Some fresh solvent, gently wipe the surface of the plate, and then put it back in the drying oven to continue drying.
Post-exposure and de-sticking media can greatly improve printability. The prepared flexible plate is best placed for 12 hours and then subjected to an offset printing so that all the residual solvent absorbed into the plate at the time of plate making can be volatilized.
In short, in actual operation, the plate-making staff of the flexographic plates must closely cooperate with the personnel of the departments of ink draft design, film production, and printing, communicate with each other, and sum up lessons learned in a timely manner so as to improve the overall level of flexographic printing. (Liang Xinyu Witlon (Hong Kong) Limited) ("The Graphic Arts Society Monthly")
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