Q&A on flexographic printing technology (2)
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How to understand the flexographic plate making process as a whole
At present, the flexible plates introduced in China are basically solid photosensitive resin plates, and the thickness of the plates is selected according to the requirements of the flexographic printing machine. The common plate thickness is 1.70 mm and 2.29 mm. The flexographic plate making process is a process of transferring graphics to a printing plate through a negative film, which mainly includes the following processes:
(1) The back side exposure of the plate with ultraviolet light is used to determine the height of the relief on the plate, ie the depth of corrosion, and to cure the substrate.
(2) The printing plate is exposed to the front side under the action of ultraviolet light to form a printed portion on the printing plate and solidified.
(3) The plate is brushed in a solvent to remove the unexposed portion of the plate to form a relief.
(4) The plate is dried in a dryer to promote the evaporation of the solvent absorbed in the plate as soon as possible, so that the thickness of the plate is restored to the original standard value.
(5) Post-exposure and de-bonding of the dried plate can further cure the shoulder and the base and improve the printing performance of the flexographic plate. The high-quality flexo plate should have a printing durability of more than 500,000 impressions, or even up to 1 million impressions.
How to calculate the amount of reduction in flexographic film
As mentioned in the last issue of Q&A, the amount of reduction 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 for calculating the amount of film shrinkage (percentage) for a flat exposure flexographic version: Reduced amount (percentage) = 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 printing length of the plate cylinder in millimeters. If the printing length of a certain product is R=235 mm, the thickness of the plate is 1.70 mm, and K=9.89 mm, the amount of reduction is 9.89/235×100%=4.21%.
When designing the ink draft, the film should be longitudinally reduced according to the above ratio, and then through the plane exposure plate making, the printed product will meet the size requirements. It should be further explained that the printing length is analyzed and estimated in actual production. The determination of R should consider many factors. For example, when the plate roller gear module is 1/8 inch, the printing length can only be increased or decreased by 3.175 mm steps; other factors also reduce the paper consumption, leaving the sticking plate.馀, 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 corrosion depths by performing a back exposure test on the platesetter. For comparison purposes, we used a method of multiple exposures of the same plate with different exposure times. Take a piece of material about 320 mm long and 100 mm wide. When performing back exposure on the platesetter, we use a red cover strip to partially cover the area, and the plate-making machine exposes multiple times, resulting in five different back exposure times. The printing plate of the area. In addition, a blank of about 20 mm is left at one end of the plate for the plate washer to hold. Suppose we choose the Dutch-made AV-500 combination plate-making machine and GRACE's 1.70 mm thick flexible plate. After partial occlusion and multiple exposures, we get 0 time exposure on the same plate. Five parts of 15, 20, 25, 30 seconds, after exposure, put on the drum of the brush plate for washing, about 7 minutes. It is then dried in a drying oven at a temperature of 50. c, the time is 1.5 hours. Finally, the plate is removed, and four different plate thickness values on the plate can be measured with a caliper, while the unexposed portion of the plate leaves only a hard polyester base.
Through the above tests, the plate making machine can be quickly familiarized, and the test results are also instructive for adjusting the corrosion depth of the printing plate. It can be seen from the above experiments that the length of the back exposure time affects the thickness of the substrate, and therefore, for different printed images, different back exposure times should be used for plate making. Still taking the 1.70 mm thick flexible plate as an example, the reverse white and large solid parts on the plate have a deeper corrosion depth (ie, the plate base is thinner), up to about 1.0 mm, and the back exposure time is the shortest; The ordinary image part has a moderate depth of corrosion, about 0.85 mm, and the back exposure time is moderate. As for the small lines and the dot part, the corrosion depth should be the shallowest, about 0.63 mm, so the back exposure time needs to be the longest.
In addition, there are two points worth noting: when the printer switches to a flexographic version of another thickness or a flexographic version of another brand, the back exposure test must be repeated. As the UV lamp is used for a longer period of time, its light intensity will be reduced. At this time, the back exposure time should be adjusted accordingly.
How to do a positive exposure test
The front exposure is also called the main exposure, which forms the printed portion of the flexographic version. Proper positive exposure and back exposure work together to achieve a high-quality printing version that meets the requirements of various graphic types. In general, under-exposure will cause the bottom of the shoulders around the graphic to be brushed off, which will affect the service life of the printing plate; excessive positive exposure will cause insufficient corrosion, which may cause paste, which will result in waste during printing.
Before performing the front exposure test, we recommend that the operator prepare a test "sample" for the flexographic version. The "sample" should contain small anti-white, ordinary anti-white and solid, ordinary graphic, small Lines and characters, dots and other basic printed graphic types, and a certain number of network lines (such as 150lpi) from 10% high to 100% of the field are reflected in the "sample".
The positive exposure test can be performed by using a "swatch" on the platesetter by partial occlusion and multiple exposures. In the early stage, we can start with the less difficult business, from easy to difficult, accumulate experience, and fully master the technology of flexo. In theory, the finer the image, the longer the positive exposure time is needed, but remember that it works in conjunction with the back exposure. For example, anti-white characters require less back exposure and positive exposure, while small dots require longer shallow exposure depths, so their back exposure and positive exposure time are longer.
After the general exploration and practice, the plate-making operator should understand the limits of his own plate making, such as the thinnest lines, the smallest characters, the most dense dots and 3% of the outlets can stand up, 50% of the outlets are white "Opening up" and so on.
What should be paid attention to when washing and drying?
As the number of times of solvent use increases, the time for brushing the plate should be appropriately extended; adjust the position between the fine brush and the plate drum according to different plates, thereby adjusting the pressure on the surface of the fine brush and the plate; if the system is particularly fine The dot should be further reduced on the basis of the standard thickness position to prevent the dot from being washed away.
The drying temperature is about 50. c, the time is about 1.5~2 hours; should be taken out several times during the drying process, check the surface of the layout for solvent spots or other edgy, use a clean cloth without fiber, soak some fresh solvent, gently Wipe the surface of the plate and return it to the drying oven to continue drying.
Post-exposure and de-bonding of the plate can greatly improve printability. The prepared flexographic plate is preferably placed for 12 hours and then tack-printed, in order to completely evaporate the residual solvent absorbed into the printing plate during the plate making process.
In short, in actual operation, the flexographic plate making staff must work closely with the personnel of the ink draft design, film production and printing departments to communicate with each other and summarize the lessons in time to improve the overall level of flexographic printing.