Flexo Printing Solutions 14 Questions (12-14)

12 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. Then dry in a drying oven at a temperature of 50. c, the time is 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.

13 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 “samples”.

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 “stopped”, and whether 50% of the dots are highlighted. "Open up" and so on.

14 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.

The drying temperature is about 50. c, the time is about 1.5 ~ 2 hours; should be removed several times in the drying process, check the layout of the surface with or without solvent spots or other hair, you can use a clean cloth can not afford fiber, dip in some fresh solvent, gently wipe The surface of the plate is then placed back in the drying oven and allowed to dry.

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.